Here's why:

* Extreme Pressure: The inner core is under immense pressure from the weight of all the layers above it. This pressure is so great that it forces the iron and nickel in the core to be solid, even though the temperature is incredibly high.

* Temperature: Temperatures in the inner core are estimated to be around 5,200°C (9,392°F)! This is hotter than the surface of the sun.

While the outer core is also very hot (around 4,500°C or 8,132°F), it's in a liquid state due to slightly lower pressure.