When an earthquake strikes, it creates horizontal forces that shake the building from left to right. Concrete is very strong when compressed, but it has little flexibility, and when stretched, as it is during an earthquake, it's liable to crack. So to create earthquake resistant buildings, designers add a flexible steel skeleton known as rebar. The steel is elastic and springs the building back into shape. Sometimes, when tension is too high, the steel may warp permanently. This is advantageous, though, because it keeps the building upright, enabling people to escape. There are other ways to make earthquake resistant buildings. Watch the video to learn more.