Here's why:

* Extreme pressure: The immense pressure at the Earth's core, estimated to be over 3.3 million times the pressure at sea level, is so great that it forces the iron and nickel to solidify, even though the temperature is extremely high (around 5,200°C).

* Unique state of matter: The inner core is sometimes described as a "solid plasma" due to the intense heat and pressure. This means the atoms are tightly packed, like a solid, but they're also constantly moving and colliding, like a plasma.

Let me know if you have any other questions about the Earth's core!