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

Carried through body happens if cancer cells enter the blood stream.
What happens if cancer cells enter the blood stream?

The pupil is the name of the black opening in the eye that lets light enter.
What is the name of the black opening in the eye that lets light enter?

Exercising regularly is a good health habit.
Which activity is a good health habit?