How do scientists know what the earth is made of?
One way that scientists can tell what the Earth is made of is by density. Since different substances have different densities, this is one way for scientists to distinguish between materials that the Earth is composed of.
Another way is by chemical properties and physical properties. For example, a scientist can tell the difference between pyrite (fool's gold) and gold by their luster, which is a physical property that distinguishes them.
Another common way is by half-life and radioactive decay. These are concepts that you may pick up in your science class, but if you don't know, then different elements have varying half-lives. A half-life is the amount of time it takes for exactly one-half decay of an element, a concept closely intertwined with radioactive decay. Since different elements have varying half-lives, this is another way to distinguish elements that make up the Earth.
I hope that helps!