A seesaw is a piece of playground equipment made of a board balanced on a center support. To play on a seesaw, you sit on one end with a friend on the other end, moving up and down.
You might also call a seesaw a teeter-totter — they are common in parks and on school playgrounds. When you sit on a seesaw, with your friend on the opposite side, each of you takes turns pushing up off the ground. When you go up into the air, your friend touches the ground, and when your friend goes up, you go down. Seesaw comes from the French ci-ça, or "this-that."