"If it moves, it's biology. If it stinks, it's chemistry. If it doesn't work, it's physics."

This is a humorous way of describing the different areas of science.

* Biology deals with living things and their processes.

* Chemistry explores the composition, structure, properties, and reactions of matter.

* Physics studies the fundamental laws governing the universe, including motion, energy, and forces.

The joke highlights how physics underlies everything, even when things go wrong.