Flexi Says: Earthquakes happen when the movement of tectonic plates causes stress to build up in the solid rock of Earth's crust and the rocks break. Earthquakes have likely been happening since early in the Earth's history after the planet cooled enough to form a solid crust. Scientists have observed similar "quakes" on other planets and moons with solid crusts, including Mars ("marsquakes") and Earth's Moon ("moonquakes").
