Since this is (also) a trivia manga I might as well mention that the "separate stomach for dessert" of course does not exist, but.. what happens is that since sugar (a high energy source) was rare back in the evolutionary days, it was a benefit to be able to gorge it all down if you found any, so for the average person (not everyone) the stomach will relax in an attempt to make room for more food if you start eating sugary stuff.