Triangles keep their shape

In your book, Maths Mela (Chapter 1), you make shapes with straws joined by clay at the corners. First make a triangle and gently push one of its sides. The triangle does not change its shape! Its three straws hold each other firmly in place.

Now make a rectangle. It has 4 right angles, one at each corner. Gently push one side and it leans over, like a pushed door frame. Its angles are no longer right angles. Two corners close up and become acute angles, and the other two open out and become obtuse angles. So a pushed rectangle has 2 acute and 2 obtuse angles.

A square does the same. Push one side and its 4 right angles change into 2 acute and 2 obtuse angles. A pentagon made of 5 equal straws changes too: its angles are not right angles to begin with (each is a little more than a right angle), and when it is pushed, its angles change.

Of all these shapes, only the triangle keeps its shape when it is pushed. That is why we say the triangle is the strongest, most rigid shape.