[Signs] It really wasn’t the water that hurt the aliens

With Signs being so old, I'd be astounded if someone else hasn't already come up with this theory, but I haven't seen it after searching so here goes.

To begin with, whole idea that “water hurts the aliens” is ridiculous on its face. The Earth is covered with water. There is water in the air. In fact there is a LOT of water in the air. There is water in the human body. There is just no way that the aliens are not going to notice that the environment and all of the inhabitants are completely toxic to them.

So what hurt the aliens? They tell us right in the movie. When the daughter is asked why she won't drink the water, she says that the water is “contaminated” and that it “tastes funny” and “tastes old”.

Contaminated water is not rare, and many of the contaminants are highly toxic. If you get your water from a well, you can often find arsenic, mercury, radium or a host of other toxins in trace quantities. If it's from a city supply it can also contain excessive chlorine. Many of these are essentially tasteless, so they might go unnoticed, but perhaps a young girl's sense of taste is sensitive enough that she can detect them. The family also lives on a farm, so it's possible that the water straight up contains pesticides. They mention that the aliens may be avoiding lakes in the area, which would also make sense if the groundwater was contaminated.

At the end of the movie they also mention that some small villages in the middle east had found a “primitive method” to defeat the aliens. It's possible that the method that they came up with was to just spray the aliens with a can of Raid or some other poison.

So that's it. It's not the water at all. It's the arsenic/RoundUp/insecticide that was dissolved in the specific water in that house that hurt them.