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

[Q]: Elements are pure substances that make up all matter.
[A]: Elements are pure substances that make up what?

[Q]: Oxygen is transferred through capillaries.
[A]: Which blood vessels is oxygen transferred through?

[Q]: The epidermis of a leaf contains openings known as stomata, through which the exchange of gases takes place.
[A]:
Which part of a leaf contains openings known as stomata, through which the exchange of gases takes place?