flex-wrap: wrap

CSS
<!DOCTYPE html> 
<html> 
<head> 
    <title>flex-wrap property</title> 
    <style> 
        #main { 
            width: 400px; 
            height: 300px; 
            border: 5px solid black; 
            display: flex; 
            flex-wrap: wrap; 
        } 
          
        #main div { 
            width: 100px; 
            height: 50px; 
        } 
        h1 { 
            color:#009900; 
            font-size:42px; 
            margin-left:50px; 
        } 
        h3 { 
            margin-top:-20px; 
            margin-left:50px; 
        } 
    </style> 
</head> 
<body> 
    <h1>GeeksforGeeks</h1> 
    <h3>The flex-wrap:wrap property</h3> 
    <div id="main"> 
        <div style="background-color:#009900;">1</div> 
        <div style="background-color:#00cc99;">2</div> 
        <div style="background-color:#0066ff;">3</div> 
        <div style="background-color:#66ffff;">4</div> 
        <div style="background-color:#660066;">5</div> 
        <div style="background-color:#663300;">6</div> 
    </div> 
</body> 
</html>
Source