A fable will do

I've generally been able to let me children know when I don't know an answer to a question. Sometimes I'll say something like this: "Well, I'm not sure why that is, but maybe..." And the remainder will be filled in with my best guess or what I know about similar situations.

If I simply say, “I don’t know,” Genevieve will reply, “Make something up, daddy.” She wants an explanation for everything, and if she can't get that then she wants a conjecture, and failing that a fable will do. She insatiably and always wants to know.

So when she is asking for the 21st reason during the fifth minute of a car ride, my choices are:
(1) to remember how I once wanted to know why for everything, or
(2) to say "This is the last one, then I'm out of why's", or
(3) to start providing stories with increasingly less connection to plausible reality, or
(4) to bang my head against the steering wheel while softly whimpering.

Number four is recommended only while at a complete stop. Other than that, they are equally effective (or ineffective) as a coping mechanism.