In hindsight, the three extremely vague rules that come with the sale of the world's cutest mogwai have always been the dumbest part of Gremlins. Considering that the consequences of not caring for the mogwai in the exact right way and within the exact right time results in utter chaos and irreparable destruction, you'd think Mr. Wing and his grandson would both be a little more hesitant about letting the general public anywhere near the little fur ball, never mind selling him to a total schmuck like Rand Peltzer.
But that's exactly what they do when Mr. Wing's grandson carelessly hands off Gizmo with little more than a rushed speech outlining the three dubious rules for taking care of him. Instead of providing a thick volume of strict guidelines to care for Gizmo and prevent a batch of murderous critters from taking over entire towns or cities (or the whole country), the kid casually reels off the fact that mogwai can't be exposed to bright light, be fed after midnight, or get wet, with no explanation as to why.
Worse still, Mr. Wing's grandson leaves plenty of room for error by being conspicuously unclear about exactly what time after midnight mogwai can start eating again, what types of light constitute as "bright," and whether or not letting Gizmo consume fluids would constitute getting him wet. There are leather handbags that come with stricter care guidelines.