That's a very good question, and one that can't be answered very easily. There are many, many different types of ways that scientists and engineers have found to create structures that are both strong and light.

The one structure that most people think about when talking about structure and geometry is a bridge. You've probably noticed that bridges aren't just a flat platform - that probably wouldn't be very strong. But bridge designers also don't fill in the entire space to create a bridge. That would take way too much building materials, but it would also make the bridge less stable. Since bridges are often designed with a curved support, it is very strong. This is because the weight on top not only pushes down on the bridge, but the supports end up getting pushed outward into the ground, and this helps add to the stability. You may have noticed that these supports are usually built in the shape of a half-circle - this is one way to make a very stable, but very light structure.