Stuff vs. Things

 There is a distinct difference between stuff and things. You can count things, but you cannot count stuff.
You can only measure stuff. For example, one could have eight bowls of cereal in the morning. “Bowls” are things, not the cereal. One could eat two tubs of peanut butter. “Tubs” are things. But, cereal and peanut butter themselves are stuff. You cannot eat nine cereals or eighteen peanut butters. Cereal and peanut butter fall under the category of stuff. (If you are talking about types of cereals or peanut butters, then types are the things, not the cereal or peanut butter itself.)
You need to explicitly apply a unit of measure to quantify stuff. The unit of measure used is the thing that can be counted and then applied to the stuff.
Other examples:

• bicycles – things
• raisins – things
• sweat – stuff
• bubble gum – stuff
• list items – things

You may say, “But, Matt! Bubble gum ain’t stuff, it’s thangs!” I would reply, “You should double check your grammar before saying something like that.”
I have a theory. I think that it may be that things can be pluralized, while stuff cannot. I will have to look into it further.