Maybe he was saving it to make a pipe?

It took me a while to figure out what was going on here. Evidently, Roommate #1 left an empty roll of toilet paper on top of the tank, which made Roommate #2 so angry that, instead of throwing the roll away himself, he grabbed a pen and paper and left a sarcastic note about a toilet paper roll having a place to live (the fucking trash can!). Then roommate #2 decided to answer that note with one of his own along with a solution to the problem in the form of a makeshift home for the empty roll.

Sources: Redditor WeasleyIsOurKing7