In the episode where Dave and Lisa are both campaigning to be boss, the side story is that Matthew is trying to get elected floor fire-martial, Bill tries to test Matthew by setting his plate of food on fire. After Matthew ignores the fire, he leaves and Lisa has to go and get the fire-extinguisher to put the fire out, and the blast of the extinguisher blows all the food content off the plate, leaving an empty plate. In the next few minutes of dialogue between Lisa and Dave, you'll notice the plate switches from food to no food several times!