Breaking what rule turns a mogwai into a gremlin?

Here's a little view into what the world was like before the PG-13 rating existed: You could release a trailer for a movie that shows a super cute creature driving around in a Barbie car looking sweet, inspiring parents to bring their five-year-olds to see it, and then have them get traumatized when said cute creature spawns hundreds of evil, violent gremlins.

Turning 40 in 2024, this 1984 horror-comedy-holiday classic was really just a lesson in responsible pet ownership, and if a few kids got permanent psychological scars along the way, so be it.

All pets come with rules, like having to clean out the litter box or take the dog for daily walks (needy) — but there's nothing quite like a mogwai. There's a whole bunch of really, really important rules, but not so important that you need to stick around listening to some old man drone on about the potential consequences.

Eventually, you'll find out that mogwais can cocoon themselves and turn into slimy, green, mean little buggers — IF you break this one rule. Which rule is it?