Seesaw.

The physics are clear Gravity and torque Bring delight to all You see it in the faces Of children young and old.

At least two must play Singularities insufficient You await a partner To begin the game That lasts as long As patience asks; Even forever If you like.

For simple machines, We are the seeds, The universe gives Forces in balance Giving and taking The yin, the yang, The you, the I.