This applet shows a proof of the Pythagorean Theorem which states: for right triangles, the sum of the squares of the legs is equal to the square of the hypotenuse.
Segments a and b are legs.
The hypotenuse is side c.
The default setting is a 3,4,5 triangle which is a Pythagorean Tripple, which means the three sides are integers. Try moving a vertex of the triangle and see how the new lengths fit into the equation. See if you can find another Pythagorean Tripple.