This is the thing that blew my mind about them when I first got mine - they actually have a built-in timer to make sure they caught something living, and thus worth digesting.

When something first lands in one of the traps and touches a trigger hair, the plant does nothing - it has to sense two trigger hairs being touched within 20 seconds for it to snap shut, and then it has to sense more hairs being triggered - five in total - in order for it to tighten even more and then start digestion.

The video above actually only shows the first stage of the trap closing - if it was 30-60 seconds longer you would have seen the trap shut tighter, so there were no gaps left, as the flies kept moving around inside.

If a trap was accidentally triggered without live prey inside (like by a rain drop), or a fly managed to get out in time, then the trap would shut but when the plant didn't detect anything still moving inside it would open again slowly - takes about a day or two.

But don't trigger them on purpose - it takes a lot of energy for the plant to close and reopen them, and if it's not getting any food from the effort it could kill the plant.

The traps themselves can last for a few months before dying off and being replaced. They can only be triggered so many times though, so once they've opened and closed a few times they become inactive and basically just serve as normal leaves for photosynthesis.