The earth began about 4.6 billion years ago together with all the other planets in our solar system.
The solar system was formed by a gigantic cloud of dust and gasses. About 6 billion years ago, this dust cloud condensed and formed what we know as the sun. All the dust and gasses left over from this process settled in a ring around the sun. Gradually, the material floating around the sun compressed and formed the planets, as we know them today.
The cloud from which the planets were formed was cold, but due to the heat generated by the process of compression and the thermal energy arising from radioactive decay, the earth became liquid.
The earth is still warm; the energy from the creation of the earth still keeps it geologically active.
The reconstruction
The inner core
At the center of the earth, 5,140-6,370 km down, there is an inner core, which is solid. It is 90% iron and nickel. This core is extremely hot, about 5,000 degrees centigrade. The reason it is not liquid is that it is subject to the enormous pressure of almost four million bars.
The outer core
Outside the inner core is an outer core, the depth of which is 2,900-5,140 km. It contains the same elements as the inner core, but as the pressure is less high, the outer core is liquid.
The inner and the outer cores total 1/3 of the weight of the earth.