The type of energy given off by fossil fuels and used to boil water is heat energy, also known as thermal energy.

Here's how it works:

* Fossil fuels (coal, oil, and natural gas) contain stored chemical energy. When burned, this chemical energy is released as heat and light.

* The heat energy from burning fossil fuels is transferred to the water. This causes the water molecules to move faster and further apart, increasing their temperature.

* As the water temperature rises, it eventually reaches its boiling point (100°C or 212°F). At this point, the water changes state from liquid to gas (steam).

So, the energy transformation involved is: chemical energy (in fossil fuels) → heat energy → boiling water.