WASHINGTON (AP) – Here are some of the key moments in Thursday night's GOP presidential debate, hosted by Fox News and Google:

Big moment:

A moderator asked Texas Gov. Rick Perry to explain how he would change Social Security -- and whether the plan could work if it was run by 50 states instead of the federal government. Former Massachusetts Gov. Mitt Romney has claimed that's what Perry wants to do.

Perry snapped back at Romney: "Now, it's not the first time that Mitt has been wrong on some issues."

Romney insisted again that Perry, in his book, "Fed Up!", says Social Security is unconstitutional and should be returned to the states. "So you better find that Rick Perry and get him to stop saying that," Romney said.

Perry had criticism of Romney's book, "No Apology," ready. In the hard copy, "you said it was exactly what the American people needed -- to have that RomneyCare given to them as you had in Massachusetts," Perry said, referring to the health plan Romney signed as governor. "Then in your paperback, you took that line out. So speaking of not getting it straight in your book, sir."

Other notable moments....