The legend itself.
In a right triangle, the square of the hypotenuse equals the sum of the squares of the legs (a² + b² = c²).
Used by rope-stretchers in Egypt to create perfect 90-degree corners for pyramids using a 3-4-5 rope.
Legs are short. Hypotenuse is long. Square them, add them, root them.
In Plain English: If you know two sides of a right angle (L-shape), you can calculate the diagonal connecting them. It connects Geometry (shapes) with Algebra (equations).
In The Real World: Navigation. If you fly 300 miles East and 400 miles North, this theorem tells you exactly how far you are from the airport.
Forgetting the Square Root! If a² + b² = 25, the answer is not 25. It is 5.