Pythagoras’s theorem is one of the most fundamental theorems of mathematics, used not only by mathematicians themselves but also in many everyday professions: through the centuries architects have used a string with thirteen knots to construct right angles. In Western culture the theorem was proved by Euclid in the first chapter of his “Elements”, but there are several other proofs, from China, for example (in “The nine chapters on the mathematical art”), and there is a more modern proof that is easier for children. I will present several of these proofs, drawing attention to the cultural differences they reflect, and I will show how easy it is with this theorem to construct irrational proportions, such as or √5 or √7. To conclude I will present a slightly more difficult geometrical exercise.