I know exactly how you feel, i have been in this situation before. It's just role playing and manipulation. It usually happens for many reasons :

• That person doesn't know the answer, is incompetent and doesn't want to come up as incompetent, hence the that's what i thought.
• If that person is asking you dumb question in front of people, and answering them right after, he might try to put you in the incompetent place, or playing games on you (the work place is full of that type of people, trying to climb on top of you).
• That person is lazy and wants you to do his job for him.
• That person is the "by hear" type of guy which means he can reproduce a sample of code but cannot adapt it because he doesn't understand how it works, just where to use it.

The good approach IMO is just to tell that guy, sorry pal, you're on your own now, you're going to have to figure it out yourself this time, I've got things to do. If the problem persists, talk to your manager about it (i know stooling is not the best but hopefully you have a competent manager that has seen snowing before :) )