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Okay... First thing to note is, food stuff is a list that's outside the function, so remove food_stuff and put it outside the function. That way when you use the function, it can be reused with other lists, and not just specific to food stuffs, the first argument will just reference the list you want to append to and the second argument is the item you want to add to the list.
So for your function, recall your lesson on list. You use the append method to add an item to a list.
So that's what you should use to solve this.
Try it again, and show us what you can come up with.
I hope this explanation is helpful