How do we know what the earth interior is made of?
The way earth quake waves travel through it.
There are two types of Earth quake waves:
Secondary, which is: "A type of seismic body wave in which rock particles vibrate at right angles to the direction of wave travel. Secondary waves cause the rocks they pass through to change in shape. These waves are the second fastest traveling seismic waves (after primary waves) and can travel through solids but not through liquids or gases." - Dictionary.com
And Primary waves are "A type of seismic body wave in which rock particles vibrate parallel to the direction of wave travel. Primary waves are alternatingly compressional and extensional, and cause the rocks they pass through to change in volume. These waves are the fastest traveling seismic waves and can travel through solids, liquids, and gases. Also called P wave. "-Dictionary.com
These help us know the content of the Earth's interior by waves not moving through liquid (which is the outer core), moving through solids, and bending through all of the above.