the power of triangles

The triangle is the strongest shape, this is because if there is any pressure on top or beneath it the triangle will only bend in the section where there is the most pressure. With most other shapes - such as a circle or square - the top would just bend and then collapse on top of the rest of the structure. But at the same time triangles are not the stiffest shape but is actually pretty flexible compared to quite a lot of other shapes, which makes it one of the most important shapes for structures like bridges.

You can try making a bridge out of straws and then see how strong it is! 

Here are some bridges with triangles.