What do you do when Friends/coworkers take fries/onion rings etc. without asking?

You are eating, then someone you know walks by (sitting by you..) just takes a couple of fries and/or some of the food you are eating WITHOUT asking, and after he/she puts it in his/her mouth, he/she makes that stupid (heh…I hope that is OK awkward smile) face…
How do you handle this situation? do you do it yourself?
Am I making a big deal out of this?
It’s not my fault that I HATE people digging through/touching my food.
If they ask me, I’d be more than glad to share my food.
But ASK first..