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

Q: Limestone is insoluble in water, so acid can dissolve it.

A: Limestone is insoluble in water, so what can dissolve it?
****
Q: Digestion begins in the gastrovascular cavity.

A: Where does digestion begin?
****
Q: Star clusters are groups of stars called that are smaller than a galaxy.

A:
What are groups of stars called that are smaller than a galaxy?
****