Here, a right angled triangle is fixed on the table. Each side of the triangle is adjoined by a square having sides equal to the length of respective side of the triangle. Except for the square adjoined to hypotenuse, fill the other two squares with coloured cubes. Take out the cubes from both the sides and fill them into the square on the hypotenuse side.

What did you observe? 
The square on the hypotenuse side can exactly accommodate all the cubes.

The science behind it :
According to Pythagoras Theorem, the square of the hypotenuse of a right angled triangle is equal to the sum of squares of the other two sides.