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Pythagoras' Law?
(lemm.ee)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. The best ones are thoughts that many people can relate to and they find something funny or interesting in regular stuff.
Counterexamples: The law of sines and law of cosines in trigonometry.