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

[EX Q]: Some viruses carry a few viral enzyme molecules within their capsids.
[EX A]: Some viruses carry a few viral enzyme molecules within their what?

[EX Q]: Competive exclusion princple explains why two different species cannot occupy the same niche in the same place for very long.
[EX A]: What explains why two different species cannot occupy the same niche in the same place for very long?

[EX Q]: The thymus gland produces t cells.
[EX A]:
What does the thymus gland produce?