system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes Earth’s core hot? Why isn’t it just a cold blob made up of metal and minerals?
gpt
Two major reasons: 

1. The earth was formed as a result of accretion. Small particles  gradually attracted each other due to gravitational forces and formed bigger objects. This process accumulated a lot of heat; when two objects collide, heat is generated. And as a majority of the heat was retained and never lost, the earth's core remained hot. 
2. However, a majority of the heat (>90%) is produced through the **decaying of radioactive elements**. The nucleus of unstable elements like U238 (Uranium) gradually break down, releasing energy.