Two diameters

In your book, Maths Mela (Chapter 7, Shapes and Patterns), we play with circles. Draw a circle with a compass and mark its centre. Draw a diameter: a straight line from one side of the circle to the other, passing through the centre. Mark its two endpoints. Then draw another diameter and mark its two endpoints. Now join the four points, going round the circle.

The shape you get is a rectangle: its opposite sides are equal and all four of its angles are right angles. Try again with a different pair of diameters. The rectangle may be long and thin, or nearly square, but it is always a rectangle.

Why? Both diameters are the same length, because every diameter of a circle is the same length. They cross at the centre, which is the middle of each one. When the two lines that go corner to corner across a 4-sided shape are the same length and cut each other in half, the shape is always a rectangle.

If the two diameters cross at right angles, like a plus sign, all four sides come out equal too, and the rectangle is a square. So this way you can only ever make a rectangle; a square is a special rectangle.