You know the old saying: Blood is thicker than water (but not as refreshing). It’s true, but blood is only thicker when it’s not dripping suspiciously from the ceiling or spraying from some slashed artery. That’s because blood isn’t a liquid like water is.
The fluids you are most familiar with flow like water does–they don’t change the way they move based on how they are moving, or what is moving through them. These are the generalized Newtonian fluids whose ability to resist deformation, their viscosity, is constant. Water is a Newtonian fluid that has the same viscosity even when a swimmer is breast-stroking through it, for example.
Blood, on the other hand with a knife in it, is a non-Newtonian fluid. Its viscosity changes depending on how much stress is placed on it. Blood in a vial is pretty thick, but swirl it around and the curtains of crimson that coat the vial’s insides thin out. It’s a so-called “shear-thinning” liquid–the more blood is agitated the less viscous it becomes.
But blood is just one type of fluid that flows unlike what you’d expect. The famously weird mixture of cornstarch in water, for example, is a shear thickening fluid, meaning that it gets thicker the more you smack it. The impact of your feet is enough to thicken the mixture to a point where you can run over it.
From there fluids get even weirder. Blood and cornstarch mixtures have viscosities that aren’t dependent on time; what matters is the amount of stress. Fluids like paint and yogurt, however, have time-dependent viscosities. The longer you stress them, the thinner they get. Printer ink does the opposite. It gets thicker the longer it’s mixed. Weird.
The oddest non-Newtonian fluid is probably one you had a lot of fun with as a kid: Silly Putty. It’s a Kelvin-Voight material, a fluid with mixed elastic and viscosity properties. Roll it into a ball and drop it, it bounces. Pull it quickly into a strand and it snaps, pull it slowly and it oozes into a wispy strand.
So yes, blood is thicker than water, but there is some point at which it flows just like it. Maybe that’s what all those Hollywood slashers figured out.