We all know that the core of Earth is in liquid form, as are most other planets as recently read. Why is this the case? Obviously, high temperatures are the cause, but what causes the high temperatures?
The central core of the Earth is not thought to be liquid, but there is a liquid layer surrounding it.
When you form a planet it involves taking material effectively from infinity and bringing it together into a deep gravitational well. You can think of things being gravitationally pulled together and their kinetic energy upon imact being converted into heat and radiation. There is therefore a large amount of gravitational potential energy which was partly radiated and partly used to make the material hot.
Subsequent to its formation, a large amount of heat in the Earth's interior is due to the radioactive decay of various isotopes in the crust and mantle - mainly uranium, thorium and potassium.
The exact balance between the two heat sources is still uncertain, but both are significant contributors.
Well actually the Earth's inner core is solid. Only the outer core is liquid iron and nickel. Now for the question what causes the high temperature, well one of the contributers is the heat that was produced by collisions and gravitational interactions during Earth's formation. Also the breakdown of heavy nuclei into lighter ones prouduces some energy to heat up Earth's core. Also some amounts of friction heat up the Earth's core to this day.