Given a scientific question, generate a correct answer to it.

Ex Input:
Potential differences from various voltage sources are necessary in order to create electrical fields, which result in what flow of charge?

Ex Output:
current

Ex Input:
For skeletal development, the most common template is what?

Ex Output:
cartilage

Ex Input:
What is formed when the ceiling of an underground cave collapses?

Ex Output:
sinkhole