I have made a pact with myself about my son. I answer his questions (as appropriate for a 5 year old) until I just can't. And then I say "I don't know." So far, he believes me.

The only time I ran into a hitch was when, while having to turn off a portable DVD player because the aircraft was landing, he loudly asked:

"But Daddy, why does the movie interfere with the nabigational [sic] equipment?"

It didn't seem like a good time to talk about that particular topic.