In 2002 the world was a different place. Mel Gibson hadn't shown us his inner anti-semite, Joaquin Phoenix hadn't fake quit Hollywood, and M. Night Shyalaman was a successful director destined to be loved by millions. These three film juggernauts teamed up to create Signs. A movie about aliens that invade our planet - but not without entertaining us first with their works of art.  

If you haven't seen the movie (or were fortunate enough to forget it) here's the trailer:


There are a number of problems with this movie. For example it stars Mel Gibson. I'll stick to the scientific flaws, though.

Water
In the final scenes we discover the aliens have a major weakness. They die when they come in contact with water. This is one of the most glaring plot holes in all of cinema history. These alien lifeforms travel untold miles to inhabit our planet and they didn't even look at a map first? The earth is more than 70% water, and humans themselves are 50-65% water. This entire alien invasion seems badly planned and ill-advised. But what's the deal with these aliens? Why do they have a problem with water?

Although the simple explanation is that water "burns" their skin, that isn't the most interesting explanation. Instead, let's make a few assumptions.

1. Their skin must be very water soluble. In other words, they will dissolve like the wicked witch the second they touch any water. This is a fair assumption and it would make a splash of water a deadly blow, just like in the movie. 
2. The aliens are in a sealed ship before attacking and all exit the ship at the same time.
3. There are millions of aliens exiting these ships at the same time.

Now, I'd like to say something extreme like that the solvation energy¹ from these aliens exiting their ships would be so great it would result in a rapid release of energy and the entire earth would turn into a giant fireball. That wouldn't happen. I thought about it from every angle, hoping it to be true, but I don't see that happening. What would happen is water molecules in the air would start adsorbing² to the aliens. Water would save the earth by a constant bombardment on these alien trespassers.

We wouldn't be in the clear, though. The aliens would die almost immediately, but as the water molecules adsorbed to the aliens the air would become less humid and the barometric pressure would lower significantly. As you know, gases move from high pressure to low pressure. When all that water vapor is rapidly adsorbed, it would create a low pressure zone. Water vapor would move from the surrounding area to reach an equilibrium pressure. If there are millions of aliens, the air currents on earth would create giant hurricane style storms around the globe. The aliens would die the second they got out of their ships, but we probably would as well...thanks a lot, water.




Crop Circles
I couldn't write about Signs without talking about...well...the signs. This main plot point in Signs is itself a big gaping plot hole. Crop circles were originally seen as proof of alien visitation. They have since been shown to be nothing more than a hoax. Although man made, there are a .  It seems statistically unlikely that aliens would attack earth using a method used in previous alien hoaxes. I suppose it could be argued that the aliens studied our planet and chose to recreate the hoax because humanity was already desensitized to the idea of crop circles. If they had paid such great attention to detail, though, you think they would have noticed all the water...

Notes

[1] Solvation energy is the energy released when a solid dissolves into water. Maybe we should invent a new unit of measurement to standardize the energy of an entire alien dissolving into the atmospheric H₂O. How about this: The person that proposes the best name for this new unit of measurement will be entered into the contest an extra time...

[2]  Adsorbing. That's your sciency word for today. You've probably used the word absorb. Adsorbing is when something sticks to the exterior surface of something.