Magma is rock so hot, deep under the ground, that it has melted and can move. It lives under volcanoes and in cracks far below our feet. When that same melted rock comes out into the air, we stop calling it magma and start calling it lava. Same stuff, different place: magma is the underground name, lava is the above-ground name. It is not an everyday word. You use it in science class, in news about a volcano, or when someone is being a little dramatic about anger (“he was magma inside”). Scientists also talk about thick magma and runny magma, because that changes how a volcano behaves — slow and sticky, or sudden and fast.