I just came across this on Wikipedia, and I am still reeling in astonishment. It's called the 'Omnipotence Paradox.' Just thought I'd throw it on here to give you guys some quick and easy ammo against theists. I'll give you the scientific idea, then the example (aka 'Paradox of the Stone').
If a being can perform such actions, then it can limit it's own ability to perform such actions. By this arguement, it cannot perform all actions, yet, on the other hand if it cannot limit it's own actions, then that is something it cannot do, and therefore, is not omnipotent.
And now for the easy version:
Could god create a stone so heavy that even he could not lift it? If so, he would then cease to be omnipotent. If not, he was not omnipotent to begin with.
Thank you for your time and patience. Another win for the Rationalists brought to you by your friendly neighborhood Atheist,