Thales' Theorem

Theorem
Let $A$ and $B$ be two points on opposite ends of the diameter of a circle.

Let $C$ be another point on the circle such that $C \ne A, B$.

Then the lines $AC$ and $BC$ are perpendicular to each other.

Proof 2
Legend has it that he sacrificed an ox in honour of the discovery.

On the other hand, some attribute this theorem to Pythagoras.