Concave describes a surface that curves inward, like the inside of a bowl or spoon. If you hold a spoon and look at the side that holds soup, you're looking at its concave side. The curve goes away from you, creating a hollow space that could hold something.

Think of a skateboard ramp: the part where skaters roll down and up is concave, curving inward to create that U-shape. A satellite dish is concave, designed to catch signals and focus them toward a receiver. Even your own hand forms a concave shape when you cup it to drink water from a fountain.

The opposite of concave is convex, which curves outward like the back of that same spoon or the outside of a basketball. An easy way to remember: concave has “cave” in it, and caves go inward into rock.

Scientists and engineers use concave shapes deliberately. Concave mirrors can focus light to a single point, which is why flashlights and car headlights use them. Concave lenses in eyeglasses help some people see better by spreading light rays apart before they reach the eye. The next time you notice something curved, ask yourself: does it curve inward like a cave, or outward like a hill?