Fluid-Mechanics

Cats do not scoop water with their tongues — they exploit a delicate balance between inertia and gravity at the air-water interface. The tip of the tongue just touches the surface; rapid withdrawal pulls a fluid column upward; the jaw closes at exactly the moment the column peaks. Reis, Jung, Aristoff, and Stocker (Science, 2010) showed that the lapping frequency of all felids — from domestic cats to lions — is tuned so that the Froude number at the tongue tip is approximately unity.

Marc-Antoine Fardin won the 2017 Ig Nobel Prize in Physics for proving, rigorously, that cats are liquid. The argument rests on the Deborah number De = τ/T: if the material’s relaxation time τ is shorter than the observation time T, the material behaves as a fluid. A cat filling a sink (De ≈ 0.008) is a liquid. A cat bouncing off a table (De ≫ 1) is a solid. The classification is not a joke — it is standard rheology, applied to an unusual substrate.