House flies have no teeth and cannot chew; therefore they only feed on liquids which they “sponge” and “suck” up to eat.
In order to eat “solid” food, the house fly regurgitates digestive fluid onto the solid food to make slurry, so it can then “sponge” and “suck” up the liquefied solid.
This is pretty disgusting when you consider that the fly that is regurgitating, sponging and sucking on your cookie might have just come from doing the same on some animal feces in the back yard, thereby spreading germs.