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

[EX Q]: Cells have four cycles.
[EX A]: How many cycles do cells have?

[EX Q]: Deep currents are caused by differences in density at the top and bottom of the ocean.
[EX A]: What is caused by differences in density at the top and bottom of the ocean?

[EX Q]: Adding the number of protons plus the number of neutrons in an atom decides its approximate mass; atomic number is another word for this value.
[EX A]:
Adding the number of protons plus the number of neutrons in an atom, decides it's approximate mass, what is another word for this value?