Speaking seriously about the Food Stamp Challenge, its heart is in the right place, but the government does not, in fact, ask anyone to try living on an average food stamp grant of $21 per person. Food stamps are awarded on a sliding scale related to your income. If you are so poor that you literally have no cash beyond your food stamps to spend on eating, your grant will be much more than $80 a month.
A better exercise is to attempt to live on the government's thrifty food plan, which is what it uses to calculate the poverty level. It's pretty spartan, but it's more than $21 for a week. Since poor families getting food stamps typically spend about 125% of the thrifty food budget, you'll have a decent idea of what their food lives involve.