diff --git a/CallFromContract.sol b/CallFromContract.sol
new file mode 100644
index 0000000..15e2ca2
--- /dev/null
+++ b/CallFromContract.sol
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+contract ContractA {
+    uint public x;
+    uint public value;
+
+    function setX(uint _x) public returns (uint) {
+        x = _x;
+        return x;
+    }
+
+    function setXandSendEther(uint _x) public payable returns (uint, uint) {
+        x = _x;
+        value = msg.value;
+        return (x, value);
+    }
+
+    function getBalance() public view returns (uint) {
+        return address(this).balance;
+    }
+}
+
+// ContractB -> ContractA
+contract ContractB {
+
+    function callSetX(ContractA _contractA, uint _x) public {
+        _contractA.setX(_x);
+    }
+
+    function callSetXFromAddress(address _contractAAddres, uint _x) public {
+        ContractA _contractA = ContractA(_contractAAddres);
+        _contractA.setX(_x);
+    }
+
+    function callSetXandSendEther(ContractA _contractA, uint _x) public payable {
+        _contractA.setXandSendEther{value: msg.value}(_x);
+    }
+
+    function getBalance() public view returns (uint) {
+        return address(this).balance;
+    }
+}
\ No newline at end of file
diff --git a/CarFactory.sol b/CarFactory.sol
new file mode 100644
index 0000000..cddb6d5
--- /dev/null
+++ b/CarFactory.sol
@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+contract Car {
+    string public model;
+    address public owner;
+    uint public cost;
+
+    constructor(string memory _model, address _owner) payable {
+        model = _model;
+        owner = _owner;
+        cost = msg.value;
+    }
+
+
+}
+
+contract CarFactory {
+    Car[] public cars;
+
+    // function create(string memory _model) public {
+    //     Car car = new Car(_model, address(this));
+    //     cars.push(car);
+    // }
+
+    // 0x447Ec763df0A9806e33130d9695a5c0a5DAe9e76
+    // 0x978a01431F9bF1d7750DE1b0b0Bd48445E8184F1
+
+    function createWithMoney(string memory _model) public payable {
+
+        require(msg.value >= 1 ether, "Not enough money");
+
+        Car car = new Car{value: msg.value}(_model, address(this));
+        cars.push(car);
+    }
+}
\ No newline at end of file
diff --git a/CryptoLeekNFT.sol b/CryptoLeekNFT.sol
new file mode 100644
index 0000000..477689a
--- /dev/null
+++ b/CryptoLeekNFT.sol
@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/extensions/ERC721URIStorage.sol";
+
+contract CryptoLeekNFT is ERC721URIStorage {
+  uint public counter;
+
+  constructor() ERC721("CryptoLeekNFT", "CLN") {
+    counter = 0;
+  }
+
+  function createNFTs (string memory tokenURI) public returns (uint) {
+    uint tokenId = counter;
+
+    _safeMint(msg.sender, tokenId);
+    _setTokenURI(tokenId, tokenURI);
+
+    counter ++;
+
+    return tokenId;
+  }
+
+  function burn(uint tokenId) public virtual {
+    require(_isApprovedOrOwner(msg.sender, tokenId), "You are not the owner or not approved@");
+    super._burn(tokenId);
+  }
+}
\ No newline at end of file
diff --git a/ERC1155.sol b/ERC1155.sol
new file mode 100644
index 0000000..fc019be
--- /dev/null
+++ b/ERC1155.sol
@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC1155/ERC1155.sol";
+
+contract GameItems is ERC1155 {
+    uint256 public constant GOLD = 0;
+    uint256 public constant SILVER = 1;
+    uint256 public constant THORS_HAMMER = 2;
+    uint256 public constant SWORD = 3;
+    uint256 public constant SHIELD = 4;
+
+    constructor() public ERC1155("https://gateway.pinata.cloud/ipfs/QmaGvW4ynPfSUNg949mXdVezsaym9nb9e1QjvQ6EAr7x8L/{id}.json") {
+        _mint(msg.sender, GOLD, 10**18, "");
+        _mint(msg.sender, SILVER, 10**27, "");
+        _mint(msg.sender, THORS_HAMMER, 1, "");
+        _mint(msg.sender, SWORD, 10**9, "");
+        _mint(msg.sender, SHIELD, 10**9, "");
+    }
+}
\ No newline at end of file
diff --git a/ERC20.sol b/ERC20.sol
new file mode 100644
index 0000000..d3e6166
--- /dev/null
+++ b/ERC20.sol
@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/ERC20.sol";
+
+contract MyToken is ERC20 {
+  constructor(string memory name, string memory symbol) ERC20(name, symbol) {
+    _mint(msg.sender, 100 * 10 ** uint(decimals()));
+  }
+}
\ No newline at end of file
diff --git a/Error.sol b/Error.sol
new file mode 100644
index 0000000..c870de9
--- /dev/null
+++ b/Error.sol
@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: MIT
+
+// require
+// assert
+// revert
+
+pragma solidity ^0.8.4;
+
+contract Error {
+    int public balance;
+
+    function deposit(int _amount) public {
+        //require(_amount > 0, "Deposited amount must be greater than zero");
+        int oldBalance = balance;
+        balance += _amount;
+        if (balance < oldBalance) {
+            revert("Impossible!");
+        }
+    }
+}
\ No newline at end of file
diff --git a/EtherUnits.sol b/EtherUnits.sol
new file mode 100644
index 0000000..a2afc0d
--- /dev/null
+++ b/EtherUnits.sol
@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.4;
+
+contract EtherUnits {
+    uint public oneWei = 1 wei;
+    uint public oneGwei = 1 gwei; // 1 * 10 ** 9
+    uint public oneEther = 1 ether;
+
+    function testOneWei() public view returns(bool) {
+        return oneWei == 1;
+    }
+
+    function testOneGwei() public view returns(bool) {
+        return oneGwei == 1 * 10 ** 9 wei;
+    }
+
+    function testOneEther() public view returns(bool) {
+        return oneEther == 1 * 10 ** 18 wei;
+    }
+}
\ No newline at end of file
diff --git a/Event.sol b/Event.sol
new file mode 100644
index 0000000..fc6efd8
--- /dev/null
+++ b/Event.sol
@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.4;
+
+contract Event {
+      event Log(address sender, string message);
+
+      function transfer() public {
+          // xxxxx
+          emit Log(msg.sender, "I send 1 ether to you!");
+          emit Log(msg.sender, "I send 2 ether to you!");
+          emit Log(msg.sender, "I send 3 ether to you!");
+      }
+}
\ No newline at end of file
diff --git a/Fallback.sol b/Fallback.sol
new file mode 100644
index 0000000..053678c
--- /dev/null
+++ b/Fallback.sol
@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+contract Fallback {
+    event Log(uint gas);
+
+
+    fallback() external payable {
+        emit Log(gasleft());
+    }
+
+    function getBalance() public view returns (uint) {
+        return address(this).balance;
+    }
+}
+
+contract SendEther {
+
+    function send(address payable _to) public payable {
+        _to.transfer(msg.value);
+    }
+
+    function call(address payable _to) public payable {
+        (bool sent,) = _to.call{value:msg.value}("hello world!");
+        require(sent);
+    }
+}
\ No newline at end of file
diff --git a/FunctionVisibility.sol b/FunctionVisibility.sol
new file mode 100644
index 0000000..97cc2ec
--- /dev/null
+++ b/FunctionVisibility.sol
@@ -0,0 +1,30 @@
+// SPDX-License-Identifier: MIT
+// private, internal, external, public
+
+pragma solidity ^0.8.4;
+
+contract FunctionVisibility {
+    uint value;
+
+    function getValue() external view returns (uint) {
+        return _getValuePrivate();
+    }
+
+    function setValue(uint _value) public {
+        value = _value;
+    }
+
+    function _getValueInternal() internal view returns (uint) {
+        return value;
+    }
+
+    function _getValuePrivate() private view returns (uint) {
+        return value;
+    }
+}
+
+contract SubFunctionVisibility is FunctionVisibility{
+    function getValueFromParent() public view returns(uint) {
+        return _getValueInternal();
+    }
+}
\ No newline at end of file
diff --git a/GlobalVariables.sol b/GlobalVariables.sol
new file mode 100644
index 0000000..cabfff5
--- /dev/null
+++ b/GlobalVariables.sol
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.4;
+
+contract GlobalVariables {
+    event LOG(address, uint);
+
+    function getGasInfo() public view returns (uint, uint) {
+        return (tx.gasprice, block.gaslimit);
+    }
+
+    function getBlockInfo() public view returns (uint, address, uint, uint, uint, uint) {
+        return (block.chainid, // current chain id
+                block.coinbase, // current block miner’s address
+                block.difficulty, // (uint): current block difficulty
+                block.gaslimit, // (uint): current block gaslimit
+                block.number, // (uint): current block number
+                block.timestamp); // (uint)
+    }
+
+    function getMessageInfo() public payable {
+        emit LOG(msg.sender, msg.value);
+    }
+}
\ No newline at end of file
diff --git a/Import.sol b/Import.sol
new file mode 100644
index 0000000..b58d29d
--- /dev/null
+++ b/Import.sol
@@ -0,0 +1,41 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+
+import "./myLibs/Car.sol";
+
+contract CarFactory2 {
+    Car[] public cars;
+
+    // function create(string memory _model) public {
+    //     Car car = new Car(_model, address(this));
+    //     cars.push(car);
+    // }
+
+    // 0x447Ec763df0A9806e33130d9695a5c0a5DAe9e76
+    // 0x978a01431F9bF1d7750DE1b0b0Bd48445E8184F1
+
+    function createWithMoney(string memory _model) public payable {
+
+        require(msg.value >= 1 ether, "Not enough money");
+
+        Car car = new Car{value: msg.value}(_model, address(this));
+        cars.push(car);
+    }
+}
+
+
+import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/ERC20.sol";
+contract MyToken is ERC20 {
+    // constructor() ERC20("cryptoleek.finance", "LEEK") {
+
+    // }
+
+    constructor(string memory name, string memory symbol) ERC20(name, symbol) {
+
+    }
+
+    function decimals() public pure override returns (uint8) {
+        return 9;
+    }
+}
\ No newline at end of file
diff --git a/Inheritance.sol b/Inheritance.sol
new file mode 100644
index 0000000..18b0436
--- /dev/null
+++ b/Inheritance.sol
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.4;
+
+contract A {
+
+    string public name;
+
+    constructor(string memory _name) {
+        name = _name;
+    }
+
+    function getContractName() public view virtual returns (string memory) {
+        return name;
+    }
+}
+
+contract B is A {
+    string public helloworld;
+
+    constructor(string memory _name, string memory _helloworld) A(_name) {
+        helloworld = _helloworld;
+    }
+}
\ No newline at end of file
diff --git a/Interface.sol b/Interface.sol
new file mode 100644
index 0000000..13ef290
--- /dev/null
+++ b/Interface.sol
@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+contract Counter {
+    uint public count;
+
+    function increment() external {
+        count ++;
+    }
+}
+
+interface ICounter {
+    function increment() external;
+}
+
+contract MyContract {
+
+    function incrementCounter(address _counter) external {
+        ICounter(_counter).increment();
+    }
+}
\ No newline at end of file
diff --git a/Library.sol b/Library.sol
new file mode 100644
index 0000000..e0936ad
--- /dev/null
+++ b/Library.sol
@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+library SafeMath {
+    function add(uint x, uint y) internal pure returns(uint) {
+        uint result = x + y;
+        require(result >= x, "Overflow!");
+
+        return result;
+    }
+}
+
+contract TestSafeMath {
+    using SafeMath for uint;
+
+    function testAdd(uint x, uint y) public pure returns (uint) {
+        uint result = x.add(y);
+        //SafeMath.add(x, y);
+
+        return result;
+    }
+}
+
+library Array {
+    function remove(uint[] storage arr, uint index) public {
+        arr[index] = arr[arr.length -1];
+        arr.pop();
+    }
+}
+
+contract TestArray {
+    using Array for uint[];
+
+    uint[] public testArr;
+
+    function testArrayRemove() public {
+        testArr.push(1);
+        testArr.push(2);
+        testArr.push(3);
+
+        // [1,2,3]
+
+        testArr.remove(1);
+
+        // [1,3]
+
+        assert(testArr.length == 2);
+        assert(testArr[0] == 1);
+        assert(testArr[1] == 3);
+    }
+}
\ No newline at end of file
diff --git a/Mapping.sol b/Mapping.sol
new file mode 100644
index 0000000..cd6cd13
--- /dev/null
+++ b/Mapping.sol
@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+contract Mapping {
+    event LOG(address, uint);
+
+    mapping(address => uint) public myMap;
+    address[] public myMappingAddr;
+
+    function setMapping(address _myAddr, uint _number) public {
+        myMap[_myAddr] = _number;
+        myMappingAddr.push(_myAddr);
+    }
+
+    function getMapping(address _myAddr) public view returns (uint) {
+        return myMap[_myAddr];
+    }
+
+    function deleteMapping(address _myAddr) public {
+        delete myMap[_myAddr];
+    }
+
+    function getTotal() public view returns (uint) {
+        require(myMappingAddr.length > 0, "The mapping has no values!");
+
+        uint sum = 0;
+        for (uint i = 0; i < myMappingAddr.length; i++) {
+            address key = myMappingAddr[i];
+            uint value = myMap[key];
+            sum += value;
+        }
+
+        return sum;
+    }
+
+    function test () public {
+        setMapping(0x5B38Da6a701c568545dCfcB03FcB875f56beddC4, 10);
+        setMapping(0xAb8483F64d9C6d1EcF9b849Ae677dD3315835cb2, 20);
+        setMapping(0x4B20993Bc481177ec7E8f571ceCaE8A9e22C02db, 30);
+
+        uint sum = getTotal();
+
+        assert(sum == 60);
+    }
+
+}
\ No newline at end of file
diff --git a/Modifer.sol b/Modifer.sol
new file mode 100644
index 0000000..bdab05e
--- /dev/null
+++ b/Modifer.sol
@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+contract Modifer {
+
+    address public owner;
+
+    constructor() {
+        owner = msg.sender;
+    }
+
+    modifier onlyOwner() {
+        require(owner == msg.sender, "Not Owner");
+        _;
+    }
+
+    function onlyCallByOwner() public onlyOwner pure returns (bool) {
+        return true;
+    }
+
+    function adminOperation() onlyOwner {
+
+    }
+
+    function withdraw() onlyOwner {
+
+    }
+}
diff --git a/MultiInheritance.sol b/MultiInheritance.sol
new file mode 100644
index 0000000..a02b07e
--- /dev/null
+++ b/MultiInheritance.sol
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.4;
+
+contract A {
+
+    string public name;
+    string public name2;
+
+    constructor(string memory _name, string memory _name2) {
+        name = _name;
+        name = _name2;
+    }
+
+    function getContractName() public view virtual returns (string memory) {
+        return name;
+    }
+}
+
+contract B {
+    string public helloworld;
+
+    constructor(string memory _helloworld) {
+        helloworld = _helloworld;
+    }
+
+    function getHelloworld() public view virtual returns (string memory) {
+        return helloworld;
+    }
+}
+
+contract C is A("A1", "A2"), B("HelloWorld") {
+
+}
+
+contract D is A, B {
+    // constructor() A("A1", "A2") B("HelloWorld") {
+
+    // }
+    constructor(string memory _name, string memory _helloworld) A(_name, _helloworld) B(_helloworld) {
+
+    }
+}
\ No newline at end of file
diff --git a/StringUtils.sol b/StringUtils.sol
new file mode 100644
index 0000000..14151b6
--- /dev/null
+++ b/StringUtils.sol
@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+library StringUtils {
+
+    function equal(string memory a, string memory b) internal pure returns (bool) {
+        return keccak256(abi.encode(a)) == keccak256(abi.encode(b));
+    }
+
+    function concat(string memory a, string memory b) internal pure returns (string memory) {
+        return string(abi.encodePacked(a, b));
+    }
+
+    function length(string memory str) internal pure returns (uint) {
+        return bytes(str).length;
+    }
+}
+
+contract StringUtilsTest {
+    using StringUtils for string;
+
+    function testEqual(string memory a, string memory b) public pure returns (bool) {
+        return a.equal(b);
+    }
+
+    function testConcat(string memory a, string memory b) public pure returns (string memory) {
+        return a.concat(b);
+    }
+
+    function testLength(string memory str) public pure returns (uint) {
+        return str.length();
+    }
+}
\ No newline at end of file
diff --git a/Struct.sol b/Struct.sol
new file mode 100644
index 0000000..721a8da
--- /dev/null
+++ b/Struct.sol
@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+contract Struct {
+    // what you need to do? task?
+    // completed? true or false
+
+    struct Todo {
+        string task;
+        bool completed;
+    }
+
+    Todo[] public todoList;
+
+    function create(string memory _task) public {
+        // Todo memory todo = Todo(_task, false);
+
+        // Todo memory todo = Todo({
+        //     completed: false,
+        //     task: _task,
+        // });
+
+        Todo memory todo;
+        todo.task = _task;
+        todo.completed = false;
+
+        todoList.push(todo);
+    }
+
+    function get(uint _index) public view returns (Todo memory) {
+        return todoList[_index];
+    }
+
+    function getDetails(uint _index) public view returns (string memory task, bool completed) {
+        Todo storage todo =  todoList[_index];
+        return (todo.task, todo.completed);
+    }
+
+    function setTask(uint _index, string memory _task) public {
+        Todo storage todo =  todoList[_index];
+        todo.task = _task;
+    }
+
+    function toggleCompleted(uint _index) public {
+        Todo storage todo =  todoList[_index];
+        todo.completed = !todo.completed;
+    }
+}
\ No newline at end of file
diff --git a/UniswapExample.sol b/UniswapExample.sol
new file mode 100644
index 0000000..921dcd6
--- /dev/null
+++ b/UniswapExample.sol
@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+// Uniswap example
+interface UniswapV2Factory {
+    function getPair(address tokenA, address tokenB) external view returns (address pair);
+}
+
+interface UniswapV2Pair {
+    function getReserves() external view returns (uint112 reserve0, uint112 reserve1, uint32 blockTimestampLast);
+}
+
+contract UniswapExample {
+    address private factory = 0x5C69bEe701ef814a2B6a3EDD4B1652CB9cc5aA6f;
+    address private dai = 0x6B175474E89094C44Da98b954EedeAC495271d0F;
+    address private weth = 0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2;
+
+    function getTokenReserves() external view returns (uint, uint) {
+        address pair = UniswapV2Factory(factory).getPair(dai, weth);
+        (uint reserve0, uint reserve1, ) = UniswapV2Pair(pair).getReserves();
+        return (reserve0, reserve1);
+    }
+}
\ No newline at end of file
diff --git a/Wallet.sol b/Wallet.sol
new file mode 100644
index 0000000..8a1f011
--- /dev/null
+++ b/Wallet.sol
@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+contract Wallet {
+    address payable public owner;
+
+    event Deposit(address sender, uint amount, uint balance);
+    event Withdraw(uint amount, uint balance);
+    event Transfer(address to, uint amount, uint balance);
+
+    modifier onlyOwner() {
+        require (msg.sender == owner, "Not Owner");
+        _;
+    }
+
+    constructor() payable {
+        owner = payable(msg.sender);
+    }
+
+    function deposit() public payable {
+        emit Deposit(msg.sender, msg.value, address(this).balance);
+    }
+
+    function getBalance() public view returns (uint) {
+        return address(this).balance;
+    }
+
+    function withdraw(uint amount) public onlyOwner {
+        payable(msg.sender).transfer(amount);
+        emit Withdraw(amount, address(this).balance);
+    }
+
+    function transferTo(address payable _to, uint amount) public onlyOwner {
+        _to.transfer(amount);
+        emit Transfer(_to, amount, address(this).balance);
+    }
+}
\ No newline at end of file
diff --git a/myLibs/Car.sol b/myLibs/Car.sol
new file mode 100644
index 0000000..9d501f3
--- /dev/null
+++ b/myLibs/Car.sol
@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+contract Car {
+    string public model;
+    address public owner;
+    uint public cost;
+
+    constructor(string memory _model, address _owner) payable {
+        model = _model;
+        owner = _owner;
+        cost = msg.value;
+    }
+}
\ No newline at end of file