A cylinder is a three-dimensional shape with two flat, circular ends and one curved side connecting them. Picture a soup can, a paper towel roll, or a tube of tennis balls: all cylinders. The two circular ends are parallel to each other and exactly the same size, while the curved surface wraps around like a wall between them.

Cylinders appear everywhere in the world around you. Batteries are cylinders. Pipes that carry water through buildings are cylinders. The drums in a drum set are cylinders. Even the pistons inside a car engine that make it run are cylinders moving up and down.

In geometry class, you'll learn to calculate a cylinder's volume (how much space is inside it) by multiplying the area of one circular end by the height. This matters in real life: engineers need to know the volume of a cylindrical water tank to figure out how much water it holds, or the volume of a cylindrical shipping container to know how much cargo can fit inside.