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: Very low density is the concentration of gas molecules in the mesosphere.

Answer: What is the concentration of gas molecules in the mesosphere?


Question: The uterus has an external opening known as vulva.

Answer:
The uterus has an external opening known as what?