This is a thread that'll fit a number of small water bottles. The thread isn't constant so that the finer threads close in near the top and the coarser threads open up near the bottom. I've tested it with a few different types of small water bottles - Publix, Great Value, Members Mark - and it fits tightly on all of them despite significant differences in thread specs.

This will not fit a two liter soda bottle type thread. I tried to get that to work as well, but couldn't. I'd love to see someone do it.