What's the difference between a solid and a liquid?

Items that are solids have tighter, more packed molecules than a liquid does. Liquids' atoms move more freely around the liquid. For example, if you had water , it moves around more than a solid would because of the way the atoms are, whereas a solid would stay as it is :)

Year  1936,  lehman predicted that the earths 'inner core' is solid. she observed that the seismic signals during her study were bent and did not pass through instead bouncing back. later it was confirmed that the earth's inner core was solid made up of nickel-iron and had a molten outer core.