Deformation of Continuous Media Paint a grid of round fluid blobs and let the flow work on them. Any motion of a blob is just three things added together: it slides, it spins as a rigid body, and it stretches along one direction while squashing the other. Simple shear (default) is a spin and a stretch in disguise. The journey: the velocity-gradient ∇u splits into a symmetric part (the rate-of-strain, which stretches) and an antisymmetric part (half the vorticity, which rotates). The white cross on the centre blob is the stretch direction; the arc is the rotation.