Ketchup is a non-Newtonian fluid, meaning that its viscosity changes under stress and is not constant. It is a shear thinning fluid which means its viscosity decreases with increased shear stress.[45] The equation used to designate a Non-Newtonian fluid is as follows: η = τ / y ˙ {\displaystyle \eta =\tau /{\dot {y}}} . This equation represents apparent viscosity where apparent viscosity is the shear stress divided by shear rate.