To add on to the OP, if you’re cleaning any kind of bong or bigger bubbler, these are great. You fill the piece up with a good amount of salt and 91%+ isopropyl alcohol, use the caps to seal it and shake the shit out of it. Prior to ordering a few packs of those, I had always just taken some saran wrap, layered it a few times over the openings and rubber banded it in place to avoid any leaks when shaking.