An ingenious professor at the University of Wisconsin has created the Science is Fun site at which a different topic is explored in-depth each week during the school year.
It is well-illustrated.
For example, most of us know that changes in pigmentation result in the beautiful fall color displays.
Most of us do not know the actual details of the chemical reactions.
This site would be especially apt for your students who are persistent in their questions to get to the bottom of things.