Bingham Plastic Fluid

You may be familiar with the popular material science concept of the "non-Newtonian fluid," also known as the "walking on custard" trick.  This is a substance whose viscosity is variable, and (for the most part) will increase proportionately to the impact of another substance on it.  In other words, the harder you hit it, the stiffer it gets.  Jump up and down on it, and it's practically a solid-- stand still, and you start to sink in.

A similar class of substances is the Bingham Plastic, named after chemist Eugene Bingham, who was a rheological pioneer.  It is similar in that its physical properties vary depending on the applied stress, but it actually acts in the opposite fashion-- under no or low stress, it acts like a solid, but if you apply stress to it, it begins to flow like a liquid.  A good example is a stiff mayonnaise-- it spreads smoothly under pressure, but left alone it retains the peaks and ridges left by the knife.