Why Bridges Stand Up

Have you ever wondered how a heavy bridge can hold up cars and not fall down? The secret is a shape called the arch. An arch is a curve that spreads weight out to the sides instead of letting it push straight down in one spot.

You can test this yourself. Lay a flat sheet of paper across two cups and set a coin on top. The paper sags and drops the coin. Now bend the paper into a small arch between the cups. This time it holds the coin without bending.

Builders have used arches for thousands of years. Old stone bridges in Rome still stand today because of them. A good shape can be stronger than a lot of extra material.