Some of the most impressive structures in the world are bridges. Bridges have to be built to withstand a variety of forces. These forces come 1 a combination of factors: the weight of the bridge, the weight of the traffic, and the strains exerted by the weather, such as wind, rain and snow. Bridges are costly to build and engineers go to a great 2 of trouble to ensure that they are safe, yet use the minimum amount of material. To be able to do this they must fully understand the properties of the materials they are going to use. 3 materials, for example, are easy to bend. They are flexible. Flexible materials can be useful in bridge-building but need to be combined with stiffer materials to support large loads. Stiff materials are difficult to bend but tend to be brittle. Glass, for instance, is a stiff material but 4 snap easily if you try to bend it. Stiff materials can be very hard to compress and are useful if used as supporting pillars in bridges. The property that is perhaps the most important in bridge-building materials is the strength to withstand large forces 5 breaking. Steel is such a strong material which is why it is often used. Concrete is a stiff material and, 6 glass, tends to snap if bending forces are applied, but it is very strong when compressed. If concrete is used to support a bending force then it must be reinforced with 7 material. Steel is usually used as it is relatively cheap and is good 8 supporting bending forces.