I have no intention of killing myself or my neighbor so no worries.
Stories like Re:Zero and Summertime Render have characters which go back in time or somehow benefit every time they die. Assassins also need to kill themselves when caught. A common way to do such a thing is to have a deadly poison in their mouth to bite when necessary. However, I'm thinking of cases where the person in question is unprepared for such an act.
I've seen in some manga/anime that biting one's tongue is a quick and easy way to die but I've looked around the internet and found many people surviving biting their tongue off, even though there does seem to be some ways and cases biting off the tongue can kill someone.
Ideally, it should be quick and highly likely to work like the commonly portrayed tongue biting. I'd also like to not rely on the environment too much, so jumping off a building and drowning are less desirable. If possible, I'd like something that can work when restrained or locked in a small room. Methods hard to detect/stop are also preferred.