This question reminds me of a Koan I heard as a kid. At least I think it was a Koan. Something like a man put a goose egg in a bottle which has a neck just large enough to accept the egg. After some time the egg hatches and the chick is cared for until it is full grown. How did he get the goose out of the bottle without harming the goose. The story I heard was written much better than that but that was the gist of it.
I have never heard "the" answer to that question but did eventually reach my own answer that seems right to me. I think this God rock question is similar in nature though not as subtle. For me, an omnipotent god could create a rock which an omnipotent god could not lift amd be damned the logic. I would say an omnipotent god could create all sorts of things that make not one bit of sense to you and me.