Original: A seesaw (also known as a teeter-totter or teeter board)  is a long, narrow board suspended in the middle so that, as one end goes up, the other goes down.
Simplified: A seesaw (also known as a teeter-totter) is a long, narrow board which is fixed in the middle, a little way above the ground. When one end of the seesaw goes up the other goes down.