This is very obvious.

When Shrek, Fiona, Donkey, and Fiona's Parents are eating dinner together, Shrek looks down at his plate and sees some sort of food. He picks one of the pieces of food, puts it in his mouth, and eats it. Moments later the plate is no longer there and instead there is a bowl. Shrek mistakes it for soup but it's really just for putting your hands in. You never see anyone switch the plate for a bowl. The plate just suddenly transforms or something!