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

SOLUTION: What is caused by differences in density at the top and bottom of the ocean?

PROBLEM: Some meteorites are made of iron and nickel and are thought to be very similar to the core of the earth.

SOLUTION: Some meteorites are made of iron and nickel and are thought to be very similar to what part of the earth?

PROBLEM: Bioluminescence is an example of metabolic type of activity that is carried out by a cell and is precisely coordinated and controlled.

SOLUTION:
Bioluminescence is an example of what type of activity that is carried out by a cell and is precisely coordinated and controlled?