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Non-Newtonian fluidics is at work here [0].

If you want to try it at home, mix cornstash with water (roughly 2 to 1) and create an Oobleck [1]. Guaranteed success with kids (and adults who remained young at heart).

[0] http://en.wikipedia.org/wiki/Non-newtonian_fluid

[1] http://en.wikipedia.org/wiki/Non-newtonian_fluid#Oobleck , http://www.google.fr/url?sa=t&rct=j&q=&esrc=s... , http://www.google.fr/url?sa=t&rct=j&q=&esrc=s... , http://www.google.fr/url?sa=t&rct=j&q=&esrc=s...

"Cornstarch in water" was my first thought when reading this title, thanks to 7th grade science class.
Same here, but thanks to Mythbusters in my case.
Another example of a non-newtonian fluid (Except, in this case, shear-thinning instead of shear-thickening) would be ketchup.
It explains some kitchen accidents...

* Tilts the bottle

It doesn't come out.

* Taps the bottom

Still nothing.

* Tap tap tap...

Nope.

* TAP!

Ketchup ocean in my plate...

It's actually quite old, the idea of using liquids as a body armor. The problem with implementation was the weight of the liquids.

From this article, it has appeared that the problem was solved, since they mention that is a lot lighter and provides more freedom of movement.

http://news.discovery.com/tech/winter-olympics-body-armor.ht...

Being used for skiers and mentions its use in body armor at the time (2010). I'm pretty sure I read an earlier article than that one as well, perhaps from 2006 or so but I can't find it.

I can't tell from this new article if they've improved the fluid, or the method of application to kevlar is the novel part.

I have a few-years-old pair of d3o motorcycling gloves; from the company that made the skier armor. It's not interwoven with kevlar, and I haven't "tested" it; but they have been around for quite a while.
Question: If it hardens when it is disturbed, does it mean that it hardens if the wearer runs, for example? This would mean you still can't properly armor joints...
I think the more force (or acceleration?) it undergoes the more it hardens. So hopefully running wouldn't affect it, but a fast moving projectile would.

Have a good day.

500+MPH impacts are vary different than simple joint movements. Think of it like this, if you slowly move your hand though water you don't feel any resistance, jump off a bridge and hit it at 100MPH and the stuff might as well be concrete.
Shear thickening fluids have a non-linear relationship between shear rate and shear stress. Schematically, it's the green line in this graph [1]. By choosing your material carefully, you can minimize shear stress at 'running' speeds, and then sharply ramp it up to protect against faster projectiles.

[1] http://en.wikipedia.org/wiki/File:Shear_rate_vs._Shear_stres...

Thanks for all the answers everyone, I get it now :)
Good... the slow blade penetrates the shield... but look down. We'd have joined each other in death. However, you did seem to finally get in the mood.
I don't think kevlar protects against blades either