An earthquake happens when the earth's plates move and shake the ground. It can be small or big and last a short or long time. We use the Richter scale to measure how strong an earthquake is. Earthquakes can be dangerous because they can break buildings and roads and cause other disasters like tsunamis. People who live in places where earthquakes happen a lot need to be ready for them by making a plan, keeping their things safe, and knowing what to do. We can't know when an earthquake will happen, but we can do things to make it less bad for us and our friends and family.