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When I used to teach environmetal ed, we would collect all the wasted food that people took on their plates and didn't eat -- called "ort" -- and weigh the whole pile of it after each meal.
At the first meal it was always, like, 17 pounds for 100 kids. We'd focus on reducing waste at each meal - taking small portions, coming back for more if you wanted, but not piling up your plate - and by the final meals, it was usually measured in ounces and fractions of an ounce.