An earthquake happens when huge blocks of the Earth's crust move and rub against each other. A great amount of energy builds up between them, and then it is suddenly set free as shaking. Most large earthquakes happen near the borders where these blocks meet. The place under the ground where the movement starts is called the focus of the earthquake. The point on the surface above it is called the epicenter. The size of an earthquake is measured on the Richter scale. The higher the number, the more energy is released. Today people cannot forecast the time of earthquakes exactly. So engineers build flexible buildings that can bear the shaking, and people practice how to take cover when it happens.

Question: Why do engineers build flexible buildings, according to the text?