When kids do things that just defy explanation, it's human nature to demand: WHAT WERE YOU THINKING!?!?! Funny thing is, in my 22 years in the field, I don't think I've ever heard a child answer this question to the satisfaction of the adult asking it.
While I understand the reason for asking- I have to say that I think it's a totally unfair question. When we ask, "Why did you do that?" or "What were you thinking?" we are setting our kids/students up for failure. Here we've gone and implied that there is a correct response to the question. As if they could say "Well, I hit my sister because she made me mad" and then you would say, "Oh, in that case... carry on!"
I'd like to suggest that asking unanswerable questions is a habit parents/caregivers should break. Instead, consider one of these responses when your children/students behave in a way that is just unbelievable:
I am so shocked by what I just (saw, heard, read) that I need a minute to think!
I'm not sure why you thought that was a good idea, but it is NOT okay!
What could you do next time to (get your point across, get what you want, etc) that will not get you in trouble?
Why do you think that behavior is not allowed?
It is never acceptable to (hit, lie, steal, put power bars in the urinals....), so how are you going to fix this?
Though these responses may never give us the satisfaction that comes from understanding the motive behind a child's behavior, they do promise a much better outcome in the long run. Many times, honestly, kids don't know why they do the things they do- so instead of dwelling on the excuse, let's help them work on a better way for next time.