But just how far she sinks depends on density. So, as quicksand is a shear-thinning non-Newtonian fluid, as soon as Professor Evil stepped out onto the pit she would very quickly start to sink. Shear-thickening non-Newtonian fluids, such as cornstarch in water (oobleck), have practical applications as materials for impact protection such as bullet-proof vests and motorbike armour, rapidly stiffening on impact, but otherwise being comfortable and flexible to wear. Just a 1 per cent change in the weight on top of the quicksand is enough. For quicksand, the yield stress is quite low-just a 1 per cent increase in the weight on top of the quicksand is enough. The level of stress or force needed to make the material flow like a fluid is known as the ‘yield stress’. There are plenty of household examples of thixotropic fluids-including toothpaste, syrups and hair gels-that appear solid but flow like a fluid with an increase of pressure or force (such as squeezing toothpaste out of a tube). Liquid mixtures with these properties are known as ‘shear-thinning thixotropic non-Newtonian fluids’ as they don’t seem to follow the normal liquid/solid rules. If left alone, the quicksand will slowly become more viscous or firmer, but with a sudden change in pressure, it rapidly liquifies. It is this special mixture that leads to some interesting properties. Natural quicksand is a mixture of fine sand, very fine micron-sized particles of clay, and water. Will Professor Evil sink all the way into the quicksand? Image adapted from: Eamon Curry CC BY 2.0 Quicksand-a non-Newtonian fluid To answer this, we need to understand exactly what quicksand is and a little of the concept of density. But does Professor Evil actually need help, or is she not in much danger? It is a Hollywood scene we are familiar with: the hero being chased through the forest and, suddenly, the evil antagonist is sucked into a large pit of quicksand only to be saved at the last minute by the kind-hearted good guy.