In Math 10 we were building up to begin adding and subtracting fractions. These are adults, so they've obviously seen and worked with fractions before, but not necessarily successfully, which is why they are taking this class. I wanted them to think about why we had to lay a foundation for adding and subtracting fractions. I asked, "What is the hard thing about adding and subtracting fractions?" What I expected to hear was, "Because you have to get a common denominator."
A student raised her hand. I called on her, and she said, in all seriousness:
Because they've already got so much going on with them!