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Please tell me the reason behind using Head & Shoulders Shampoo in the movie Evolution (2001).

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    Do you mean the in-universe reason or the reason why the filmmakers chose head and shoulders? – user7812 May 4 '15 at 15:14
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Ah, the selenium!

One of the ingredients in Head & Shoulders is selenium, an element on the periodic table.

The active ingredient in Clinical Strength Head & Shoulders is selenium sulfide 1%

Ignoring the fact that a great number of elements on the periodic table are toxic to humans, the film decided to pick arsenic off of the table and then made this leap of scientific logic:

Humans are carbon-based life-forms, and if arsenic is toxic to us, the "scientists" in the film use the periodic table to decide that the alien life-forms, which are nitrogen-based, must similarly find selenium toxic:

Selenium hypothesis

I think the film is ridiculous and haven't seen it in years but this is the one thing I remember about the film.

Here's a reference (and the source of the image) if you want it:

David Duchovny, much by chance and luck discovers that since humans are carbon based and arsenic is deadly to humans, the aliens are nitrogen based and so if you follow the same pattern (2 down 1 over) on the periodic table, the aliens should have a deadly reaction to selenium (which apparently is found in conditioner).

And from the Wikipedia article:

Looking at the positions of nitrogen and carbon on the periodic table, Ira theorizes selenium might be poisonous to the aliens, since they are nitrogen-based, as arsenic is poisonous to Earth's carbon life. Ira's dumbest students Deke and Danny (Ethan Suplee and Michael Ray Bower) recall selenium sulfide is the active ingredient in Head & Shoulders, so the team procures a firetruck and fills it with the shampoo. Ira also gives the Donald brothers an A, much to their excitement.

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    This comprehensively covers their rationale in the movie. – DarthBotto May 4 '15 at 7:42
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    Nice. So when we expose those aliens to Neon, we get the same reaction as with humans to Fluorine, i.e. they burst into flames? Quick, grab some of those fluorescent tubes... (Just to point out just how ridiculous this "reasoning" is...) – DevSolar May 4 '15 at 14:21
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    I don't remember much about the movie, but what I do remember is that it wasn't serious, so the ridiculosity levels aren't particularly important... – CGCampbell May 4 '15 at 18:59
  • @CGCampbell, I'll second that. It was ridiculous, but I think that was the point. And I didn't know that 'ridiculosity' was a word. – Reed May 5 '15 at 0:25
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    I like to think of it as a ridiculosity scale from Liam Neeson to that Chinese guy in the Hangover series.... :) You know... 0 to inifinite – CGCampbell May 5 '15 at 1:59

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