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

Deep currents are caused by differences in density at the top and bottom of the ocean.
What is caused by differences in density at the top and bottom of the ocean?

Pressure and vibration are examples of physical type of stimuli.
Pressure and vibration are examples of what type of stimuli?

320 million years many years ago did early amniotes diverge into two groups.
How many years ago did early amniotes diverge into two groups?