The mysterious extra stomach dessert-lovers possess that allow them to eat dessert even after the "regular" stomach is completely full of non-dessert food.
The dessert stomach appears in childhood, and typically gets smaller as one ages, though this is not always the case.
MOM: Finish your dinner.
CHILD: *Groan* I can't! I'm so full, I think I'll barf
if I eat another bite.
MOM: Well, I guess you're too full to eat dessert.
CHILD: I'm not too full! My dessert stomach is empty!
(child proceeds to eat entire dessert portion)