[Greek]. Thixotropy is a property shown by some non-Newtonian fluids and describes the effect of decreasing viscosity at a constant shear force. The opposite behaviour of decreasing viscosity at a constant shear force is called rheopexy. Thixotropy and rheopexy are time-depending effects. Due to this the viscosity does not return immediately to its original value after removing the shear force, but after a certain time of relaxation. Example for thixotropy are ketchup and water-sand mixtures.