One Stick — Why are triangles the strongest shape, and why do bridges and towers use them?

A square frame can fold flat when you push it sideways, because its four corners work like hinges. A triangle cannot change shape at all unless one of its sides breaks - so adding one stick corner to corner turns a wobbly box into something you can stand on. That is why bridges, cranes, pylons, roofs and bike frames are full of triangles, and why a deckchair, which is built to fold, has none in it at all. Put up a market stall, knock it flat with the wind, find the one stick that saves it, watch a fat steel bar lose to a thin cane, and then take the stick out again to fold the whole thing onto the cart.