Earth's structure is like a layer cake with different layers. It has three main layers: the core (center), the mantle (in the middle), and the crust (the outer layer we live on). Plate tectonics is the idea that the Earth's outer shell, or the crust, is divided into big pieces called tectonic plates. These plates are like puzzle pieces that float on the semi-liquid mantle underneath. They can move around and interact with each other. When they do, it can cause things like earthquakes and the creation of mountains. It's like a giant jigsaw puzzle that's always changing.