One important source of data is the waves that travel through the earth from earthquakes. These waves come in two main types, compressional P-waves and shearing s waves. After a big earthquake, we can find places in the world where no S waves were detected, from this we have been able to conclude that the outer part of earth's core is a liquid, because solids transmit S waves, but liquids do not. We can use the size of the area where no S waves occur to calculate the size of the outer core. We can find the inner core in similar but more complex ways, involving the conversion of S waves to P waves and then the refraction of those waves. These days, even more information is learned by using the same sort of computer data processing that physicians use to do CAT scans with x-rays. Geophysicists can collect earthquake waves coming from many different sources into an array of stations and then use the patterns of arrivals of different kinds of waves to do "seismic tomography" of the earth. It is important to remember though, that seismic waves only give data about density and rigidity.