system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: the physics behind non-Newtonian fluids

Hello all,

I know *what* a non-Newtonian fluid is and I know that either the shear rate or shear stress can affect the fluid's viscosity, but I'm not sure *why* they act as they do, on a molecular level.

My best guess would be the shape of the molecules themselves, but that's purely conjectural.

If anyone can tell me more or direct me to an ELI5 friendly website where I can learn more about the physics interactions between molecules in non-Newtonian fluids, I'd be most appreciative :)
gpt
ELIF answer:

Think of sand. It's soft and flows when it's wet, but if you squish it down and wait for it to dry, it's hard and crumbly. 

Non-newtonian fluids are like sand that absorbs and dries out REALLY fast. So if you apply pressure, it gets harder like you squished out the water.