I hope you enjoy it as much I did once.
Step 1: The Puzzle
You are given a circle card, a rectangle card, and a pencil. Using only the three objects,and without folding or tampering them, mark the diametric line on the circle.
Step 2: The Property
Any angle made at the circumference by the diametric line will be 90°.
Step 3: How to Do It?
As each vertex of a rectangle is a right angle, the angle made at the circumference is a right angle. Following this, the line that makes it turns out to be the diameter.