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

Interstitial fluid are all hormones secreted into before entering the circulatory system.
Where are all hormones secreted into before entering the circulatory system?

The human eye detect can detect three colors of light.
How many colors of light can the human eye detect?

When the core of a star begins to run low on hydrogen, it contracts.
What happens to the core of a star when it begins to run low on hydrogen?