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.