Indirect observation: Temperatures inside the Earth

Questions for thought

1. What personal observation makes you think that the Earth might be hot inside?
2. Heating requires an energy source. What kinds of energy heat the inside of the Earth?
3. At what depth is the interior of the Earth totally molten?
4. Is it totally molten below the moving plates?
5. Is it toally molten below midocean ridges?
6. At what temperature does granite melt at the Earth's surface?
7. At what temperature does glass melt at the Earth's surface?
8. At what temperature does bronze melt at the Earth's surface?
9. At what temperature does titanium melt at the Earth's surface?