Plagiarism and George Washington

I had to cover plagiarism today with little time. I found this quick quiz, which worked well and wasn't too boring. It's tricky at times. Obviously an essay bought on cheathouse.com is plagiarized. But what about sentences that are common knowledge? Just don't copy an exact sentence. Anyway, the quiz tested your knowledge or what was copied and what was legit. One page had a picture of George Washing and it said he is the father of our country. It was plagiarism! Why? Because you have to say where you found the picture!