instruction:
Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
question:
Deep currents are caused by differences in density at the top and bottom of the ocean.
answer:
What is caused by differences in density at the top and bottom of the ocean?

question:
Mucous does the esophagus produce for lubrication.
answer:
What does the esophagus produce for lubrication?

question:
The atom is the representative particle in pure metals.
answer:
What is the representative particle in pure metals?