An earthquake is a sudden shaking or trembling of the ground caused by movement deep inside the Earth. To understand earthquakes, you need to know that Earth's outer layer, called the crust, is not one solid piece. Instead, it's broken into huge pieces called tectonic plates that fit together like a giant jigsaw puzzle.
These enormous plates are always slowly moving, but they usually move so slowly you can't feel it. They might move just a few inches each year, about as fast as your fingernails grow. Sometimes these plates get stuck as they try to slide past each other, rub against each other, or push into each other. Pressure and energy build up at the edges where they're stuck, kind of like when you bend a stick until it snaps.
After many years, the pressure becomes too great and the plates suddenly slip or break free. When they finally move, they release all that stored energy at once. This energy travels through the ground in waves, making the Earth shake and causing an earthquake. The place underground where the earthquake starts is called the focus, and the place on the surface directly above it is called the epicenter.
Scientists use special instruments called seismographs to detect and measure earthquakes, even small ones that people can't feel. The strength of an earthquake is measured using numbers on a special scale. Small earthquakes happen somewhere in the world almost every day, but people rarely notice them. Large earthquakes are much more rare but can be very destructive, knocking down buildings, cracking roads, and changing the landscape.