[mrsmith2211 (OP)]

I guess I am confused, I have read the technobabble, but it is not clear what makes it newtonian or non newtonian.
Water, is a Newtonian fluid
cornstarch and water is non newtonian.
Now I have heard if you hit water hard enough it is like hitting concrete, so is it just a level of viscosity with an arbitrary cutoff point regarding force implied? Is not skipping a stone an applicable demonstration of water's non newtonianness?

[Colin2B]

Not sure what technobabble you have read, but the concept is quite straight forward.
In a Newtonian fluid the amount of stress you apply is linearly proportional to the strain. Put simply, this means that if you hit a Newtonian fluid with 10 times the force you hit it previously you would feel 10 times the resistance. However with a non-Newtonian fluid the relationship is nonlinear so you might only have to hit it twice as hard to get 10 times the resistance.
Apply enough stress and both will feel like concrete, but if you fall into custard you don't need to fall so far to get the same effect.
So, don't go tombstoning into any custard lakes 

[alancalverd]

Obviously, hitting water, however hard, is not like hitting concrete, but if you hit a target with something soft like a human being, at some speed the damage to the projectile will be just as fatal regardless of the target - you just get more dispersed over concrete..

[jeffreyH]

How do you get that experiment past health and safety?

[alancalverd]

Who? What?

[RD]

...don't go tombstoning into any custard lakes 

Walking on custard swimming-pools is OK, if you do it briskly ...

https://youtu.be/BN2D5y-AxIY?t=2m22s

...  with a non-Newtonian fluid the relationship is nonlinear so you might only have to hit it twice as hard to get 10 times the resistance.

Some non-Newtonian liquids become less viscous under pressure, rather than stiffer

[Colin2B]

Some non-Newtonian liquids become less viscous under pressure, rather than stiffer

Yes. I was trying to simplify things for the op, but the interesting thing is that while Newtonian is defined as only one option - linear through origin - non Newtonian is open to any other relationship covering plastic deformation and some where the slope is straight line but does not go through origin.