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

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

During the early paleozoic, the amount of carbon dioxide in the atmosphere was much greater compared to today.
During the early paleozoic, the amount of carbon dioxide in the atmosphere compared to today was?

Poles parts of the planet does solar energy have difficulty reaching.

What parts of the planet does solar energy have difficulty reaching?