Kelvin-Helmholtz instability arises when two fluid layers slide past each other at different velocities — the shear creates billowing vortices seen in ocean waves, Jupiter's clouds, and the solar corona. Rayleigh-Taylor instability occurs when a denser fluid sits atop a lighter one; tiny perturbations grow into mushroom-shaped plumes, governing supernova dynamics and inertial confinement fusion. Click the canvas to add perturbations.