Easy. Tell your granddaughter to bring in a pitcher. Any pitcher will do, but a clear pitcher, glass or plastic, would probably be most appropriate. Tell your granddaughter to fill the pitcher with water and to pass the filled pitcher around he class. Tell her to ask the students what was in the pitcher. Of course they wall all say, "Water."
Then tell her to explain to the class that saying water, or wah wah, is the Miracle from which the play, The Miracle Worker gets its name. For it is when Annie Sullivan and Helen are outside re-filling a pitcher of water at the pump that Helen first makes the vital connection between what is being spelled by Annie in her hand and the real substance of the cold, wet water she feels. It is then that Helen learns to communicate: to understand what is being told to her and how to tell others what she thinks in her sightless, silent world.
Technically, water is not exactly a food, but, for demonstration purposes, it will suffice... and may well earn your granddaughter an A.