There are many ways scientists can gather evidence to learn about Earth's interior including direct evidence from rock samples and indirect evidence from seismic waves.  Below are some resources to help you understand how scientists infer the different layers of Earth's interior:
1. Seismic waves and earthquakes
2. Volcanoes
3. Principles of density
​4. Earth is magnetic

Can we dig to the center of the Earth?

1. What layer of the Earth were scientists able to dig to?
2. What percent of the way did scientists dig to the center of the Earth?
3. What are two reasons why scientists couldn't dig any further?

So if we can't dig and "see" past the crust... 
how do we know what is inside Earth?

Evidence #1
Earthquakes & Seismic Waves

1. What happens to earthquake waves when they reach different layers inside Earth?
2. What does a P wave shadow zone tell scientists about what is inside Earth?
​3. What does the lack of S waves from earthquakes tell scientists about what is inside earth?

How do bats and people "see" without eyes?
This is just like how earthquakes help us "see" inside Earth.

1. How do bats "see" without eyes?
2. How is this like earthquakes waves and the layers of the Earth?

1. How does using sound to see similar to how we "see" what is inside Earth?

1. What happens to P waves from earthquakes when they reach liquids inside Earth?
2. What are "shadow" zones and what do they tell scientists about what is inside Earth?

Evidence #2
Volcanoes

1. What do volcanoes tell you about what is below the surface of Earth?

Evidence #3
​Density

What is density exactly?

1. What are the most dense elements that make up Earth?  Where are they located?
2. Why is the inner core solid?
3. Why is the outer core liquid?

Evidence #4
​Earth is Magnetic

1. What does a compass tell us about the Earth?