The complete, correct, and hopefully clear answer.
Can God make a rock so big He can't lift it? This is an interesting question that is sometimes used to attempt to show that the idea of an omnipotent being is logically incoherent, like a "married bachelor" or a "round square."
Answering this question requires us to define the word "omnipotent," it will also be helpful to examine another, similar question.
Omnipotence is defined (by all notable philosophers except Rene Descartes) to be the ability to actualize any logically possible state of affairs. A Christian theist like myself who believes in an omnipotent God does not believe that God can do logically impossible things like create a married bachelor or a one dimensional doughnut. This may seem to make God less great, but I think a close examination reveals that this is not the case because these aren't really "things" that we're saying God can't make, they're just words put beside each other that never refer to an actual thing.
Now let's examine a similar question. What happens when an unstoppable force meets an immovable object?
The answer to this question is that we find out who is lying [edit: in the above case it was Peyton]. Because either the force is not unstoppable, or the object is not immovable. As Isaac Asimov explained, it is logically impossible for both an immovable object and an unstoppable force to exist in the same universe at the same time. Because the question assumes that both these things exist in the same universe at the same time, the question is logically incoherent and is thus, not really a question. But it looks like a question! you might say... Yes, but it's not a question, any more than "Can shmoggle airplane toothepaste?" is a question.
The question "Can God make a rock so big He can't lift it" is a similar "not-question" because it assumes that two logically incompatible things could be simultaneously true. It assumes that:
1. A Being that can lift any rock exists
2. A rock that cannot be lifted by the above Being possibly exists
Given that God is omnipotent, it follows that there cannot be a rock so big that He cannot lift it. The idea of such a rock is logically incoherent given God's ability to lift any rock. Therefore the simple answer to this question is "no, God cannot do logically impossible things," with the additional comment that these aren't really "things" anyway, they're just a series of words placed beside each other.
If you're still a little fuzzy check out Gary Burger's answer here and the lovely YouTube video (it really is quite good, better than this post I think) from YouTube entity misterD418 here.
Btw God also "can't" make a pebble so small He can't find it
...or a crossword puzzle so difficult He can't solve it
...or a smell so bad He can't bear to go near it