Whether you are looking up at the clouds or watching coffee and milk swirl in your cup of cappuccino, you are witnessing the complex patterns that naturally occur as different fluids interact with each other. Here we see the pattern that occurs as air is injected into glycerine (a viscous substance used in a wide range of applications). The mismatch in viscosity between these two fluids prompts the sudden growth and splitting of air passages that branch into what to us looks like the petals of an exotic flower. Less poetically, this phenomenon is an "interfacial instability" known in the field of fluid dynamics as "viscous fingering," a name inspired by the finger-shaped air passages.