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

Q: Axillary buds in the axils of leaves and stems give rise to branches.

A: What in the axils of leaves and stems give rise to branches?
****
Q: Digestion begins in the gastrovascular cavity.

A: Where does digestion begin?
****
Q: The lower density of warm air causes it to rise above cold air.

A:
What property of warm air causes it to rise above cold air?
****