Such a simple answer.
The supposed question might have been, "have you eaten? Are you hungry? Do you need anything from the snack shack?"
To respond with, "I'm good," and then add the simple sentence, "I had a Pop Tart," is to let everyone know that you are adequately provisioned. Any further commentary or exchange becomes unnecessary when you give the final, declarative answer, "I had a Pop Tart." It says it all, doesn't it? It says, "hey, man, no need to trouble yourself on my account."
Our culture gets pretty complicated. But, I think everyone can understand the simple beauty of saying why you're good. A Pop Tart can propel you into the day, and keep you going for hours.