Why is triangle the most strongest polygonal shape?

Answer 1

The rigidity of the triangle derives from the fact that you cannot change any angle in a triangle without also changing the length of at least one side; in any other type of polygon, it is possible to alter the angles of the polygon without changing the lengths of the sides. So if you have a physical object in the form of a triangle, the sides are not going to change their length unless you break them. Other shapes can fold up, but triangles won't.