I used to get that question a lot when I was a kid. I always politely said "no thanks".
It turns out my face always said "Are you fucking stupid? I understand how to heat things" while I was trying to be a little polite. I found this out when I visited my parents recently and found out that, while I'm still trying to be polite, my face still tells my mom that I'm capable of deciding on the temperature of my food.
My face says things that I don't mean to say even when I'm trying not to be an ass.