A seesaw is a playground device made of a long board balanced on a pivot point in the middle. One person sits on each end, and they take turns pushing off the ground to make the board tilt up and down. When you push down and launch your friend into the air, you're on the ground. When they push down, you rise up. It only works when both people cooperate: the fun comes from the back and forth rhythm you create together.

The word also describes any situation that goes back and forth between two states. A close basketball game might seesaw between the teams, with first one leading, then the other. Your mood might seesaw between excitement and nervousness before a big performance. Stock prices can seesaw up and down throughout the day.

Whether you're talking about playground equipment or a back-and-forth situation, the image is the same: steady, alternating movement between two positions.