Seesaw: is a board which is balanced on a fulcrum. When two people sit at either end of a seesaw, they can use their respective weights to move their ends of the board up and down, causing the person on the other end of the board to move. Seesaws commonly appear in playgrounds, because they are popular children's toys, and sometimes adults indulge in a bit of seesawing as well, because it can be deeply enjoyable when the two users reach a rhythm.