Let’s look at a simpler question. If you don’t know what your friend’s bed is made of, what do you do? Easy–jump on it! If it doesn’t shake very much, it’s just a normal bed. If it shakes a lot, you know the bed is filled with water.
These are examples of waves. With the normal bed, the waves are really slow and short, while a water bed’s waves last for much, much longer.
We can’t jump on the earth hard enough to produce waves but we already have something that does produce giant waves. What shakes the earth enough to produce waves (both on land and water)? Well an earthquake does. When an earthquake occurs on one part of the world, the wave is so powerful that it goes through the Earth and is felt on the opposite side (not by people, but by sensitive equipment). Scientists measure how long it takes for this wave to be felt on the opposite side and see how powerful the wave is.
Doing this, there are 2 main possibilities:
1. If it takes a really long time to get to the other side and if the wave is not very powerful, scientists know that something solid is in the center.
2. If it doesn’t take a long time and if the wave is still powerful, we can tell that there’s something that’s slightly liquid in the center.
Using this, scientists have determined the earth is made of different kinds of “liquids”. One part of the Earth’s inside is like a milkshake. It can still flow like a liquid, but it’s not as smooth as water.
The part below that is very very liquid and it flows like water does.
The section in the very center is a solid ball. This is called the inner core.
Every time an earthquake occurs, scientists all across the world use it to find out how these waves travel and that helps us see what’s on the inside of the Earth.