Oh so many to chose from...
Having explained the meaning of "your eyes are bigger than your stomach" a few days previously I was amused when DS (then 2) told me that "my brain is bigger than my stomach". "really" was my reply. DS "oh yes- I think that I can eat more than I can"
DS at 3 "I really need some chips". Me "how can you be hungry already. You just ate a huge plate of food not that long ago". DS "it was 2 hours ago and all that food has now moved from my stomach to my small intestines." How could I argue with that!