We started the Introduction with this quote by Rutherford:"...It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." Now you know what he was talking about!
He was referring to the experiment where he bombarded positive charges at the foil. We discussed that most of the positive charges went by, but, a few rebounded.
We know that some of the positive charges rebounded because they hit the protons in the atom. Rutherford expected that positive charges weren't concentrated in protons, but, were spread around. So, he didn't expect any protons to rebound so strongly. When they did, he was extremely surprised and made the above remark.