What causes a cloud to form?  Most people could tell you that a cloud is water condensed in air.  A simple answer for a simple question.  But that answer doesn't cover it.  What actually causes the process?  Water condenses on suspended particles of dirt or dust in the atmosphere, with favorable humidity and pressure conditions.  But does that answer even do the process justice?  Different kinds of clouds form at different altitudes from entirely different weather systems and conditions.  Now you've entered meteorology.  One step in any direction opens completely new doors: the chemical reactions involved in cloud formation and chemical composition are based in chemistry; the ability for earth's atmosphere to be suitable for clouds that causes the entire process is based in geology; the hydrological processes that govern the water cycle from ground to air are based in fluid mechanics.  And on.  And on.  And soon you realize, a simple question no longer has a simple answer.
The amount of knowledge needed to completely understand the universe is as vast as the universe itself.