Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Oil is the most commonly used source of energy in the world.
What is the most commonly used source of energy in the world?

A skeleton that consists of hard, mineralized structures located within the soft tissue of organisms is called endoskeleton.
A skeleton that consists of hard, mineralized structures located within the soft tissue of organisms is called what?

Removed happens to energy when work is done by a system.

What happens to energy when work is done by a system?