@extend - makes it easy to borrow the CSS rules from one element and build upon them in another
// below block of CSS rules style a .panel class. It has a background-color, height, and border
.panel{
background-color: red;
height: 70px;
border: 2px solid green;
}
// now you want another panel called big panel, with the same base properties but also needs a width
// and font-size
.big-panel{
@extend .panel;
width: 150px;
font-size: 2em;
}