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

[Q]: All enzymes have an optimal temperature and ph.
[A]: All enzymes have an optimal temperature and what?


[Q]: Just as salty taste involves perception of sodium ions in saliva, sweet taste correlates with presence of glucose.
[A]: Just as salty taste involves perception of sodium ions in saliva, what taste correlates with presence of glucose?


[Q]: Plane mirrors work because the light rays create a virtual image behind the mirror.
[A]:
Plane mirrors work because the light rays create what behind the mirror?