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

[Q]: Line graphs are especially useful for showing changes over time.
[A]: Line graphs are especially useful for showing changes over what?

[Q]: Transpiration is when plant roots absorb liquid water and release water vapor into the atmosphere.
[A]: What is it called when plant roots absorb liquid water and release water vapor into the atmosphere?

[Q]: Carbon dioxide does not rise like other gases because it has a density greater than air.
[A]:
Carbon dioxide does not rise like other gases because it has a density greater than what?