Earthquakes happen when two pieces of the Earth's crust suddenly slip past each other. The two pieces of the Earth's crust are like two puzzle pieces that fit together. When they suddenly slip past each other, it causes an earthquake.

For a five-year-old, you can explain it by saying that earthquakes happen when two pieces of the Earth's puzzle suddenly move and shake. This shaking can make the ground shake and can cause buildings and other things to break.

For an undergraduate engineering student, you can explain that earthquakes happen due to the sudden movement of two pieces of the Earth's crust along a fault line. This movement releases a large amount of energy, which is then transferred through the Earth's lithosphere in the form of seismic waves. These seismic waves cause the ground to shake and can cause severe damage to structures if they are not designed to withstand such forces.
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