Vinsanity,
As romeo correctly said, they usually don't really require an answer.
However, if they are specifically asking you in a way that is not in awe but rather, in actual curiosity, then you have a problem.
It would then sound like you're having a presentational issue on two levels. Firstly: a deck vanishing is an amazing thing, so reactions ideally should be more of an exclamation than a question. The fact that the question is asked, and that they expect an answer, suggests that your routine isn't justified well enough. Or, more specifically, why is the deck vanishing?
This is a problem I feel which is inherent in a lot of magicians' magic, and often their justification is, "Because I can" - which sometimes works, but sometimes, like now, it doesn't. If the magic is justified, and everything makes relative sense, then that question shouldn't be asked, they should simply focus on the moment, which brings me to my next point...
The second idea is this: a deck vanish is inherently problematic because it never appears again in this instance. Therefore, you have to make sure that your presentation builds up the effect well enough that the deck vanish is understood to be the climax, rather than a penultimate stage.
Again, you shouldn't be looking to answer the question, though.