As rife with trite, watered-down and vapid statements as popular Christian culture can be, we should by no means assume that we have the monopoly on these sorts of things.  If you begin talking with one or more people who are anti-Christian (as opposed to simply atheist or even agnostic), you're apt to run into some of these equally trite, pithy little statements that can stop an unprepared person of faith in their tracks, yet really aren't as bamboozling as they appear to be.

One of these is the conundrum - Can God make a rock so big that He can't lift it?

The answer is no, and this does not mean that God is not omnipotent.  It means that attempting to describe a situation that is not logically possible is violating the laws of reason.  It's playing dirty while sounding innocent.  If you aren't familiar with philosophical terminology or precepts, you're left scratching your head, knowing in your heart that the answer must be no, but unable to determine why that ends up making God look less than all mighty.

The link above is a good, succinct explanation of why this question is invalid, because it proposes a rational impossibility.  I could summarize it, but that's sort of redundant, isn't it?