It seems that Joe Dante (known for previously helming 1981's "The Howling") and writer Chris Columbus (who would go on to direct the first three Harry Potter movies) didn't notice this. In their defense, though, a mogwai is just about the most complicated and high-maintenance Christmas present in history, so there's a lot to keep track of.
The adorable little pet given as a gift in the 1984 holiday-horror classic "Gremlins" comes with a list of strict rules: Keep him out of the light, don't get him wet and don't feed him after midnight.
The water thing is indeed a little problematic, but maybe not for the reason you point out.
Yes, they do go traipsing about in the Upstate New York snow, but that's when they're in their ugly green gremlin form, not when they're cute little mogwais. We don't know that the water rule applies to gremlins in the same way (even though the light rule definitely does).
However, there are at least two spots when Gizmo, the original mogwai (voiced by latter-day game show host Howie Mandel), cries. There's no explanation for why his tears don't set off another multiplication fit. And it's worth noting that the other rules aren't exactly infallible, either.
The don't-feed-after-midnight thing is tricky. Midnight local time? The film suggests Gizmo was imported from China -- at what point did his internal clock switch to New York time? And at what time does "after midnight" end?
As for the other rule, how much light is too much? Like time, light is relative.
Fortunately, it seems that we, as a culture, have been able to look past these flaws -- "Gremlins" is now a beloved campy horror classic, and for some a holiday institution.
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