My grandfather has this thing that he does and it is so funny. He tries to bob his head up and down without you being able to slap his head. He convinced Justin to do it with him this summer and here is the proof. The real question is not whether or not you are quick enough to catch Grampa's head, but rather, can you stop laughing long enough to try! My cousin Nate was taping it.
I just found it today and thought you might enjoy watching it!