We've been living alongside cats for thousands of years but it's taken until now to fully understand how they perform one of the most basic of tasks - drinking.

All cats, from the mightiest wild tiger to the laziest fat lapcat, drink with a very subtle action that takes advantage of a balance between gravity and inertia; the tendency for something to keep moving in the same direction unless acted upon by an external force.

How do cats lap? Cutta Cutta is about to take a drink.Back in the 1940s, MIT engineer Doc Edgerton filmed a domestic cat lapping at milk.  We've known since then that they form a distinctive shape with their tongues, curling the tip of the tongue backwards so the top, or dorsal, side of the tongue is the first to hit the water.

Writing in the journal Science this week another MIT researcher, Roman Stocker, and his colleagues have used high-speed video footage of domestic cats including Stocker's own pet, Cutta Cutta, along with a range of big cats in local zoos.  Using data from these observations they were then able to build a robotic version of a cat's tongue to really get to grips with the mechanisms involved.

When the tongue comes into contact with the liquid some of the liquid will adhere to the surface of the tongue.  As the tongue is retracted, it pulls a column of liquid along behind it.  Here's where the elegant physics comes in to play.  The column is pulled up by inertia and simultaneously pulled back down by gravity.

Interestingly, surface tension and viscosity seem to play little or no role, and as the tip of the tongue has none of the distinctive hair-like projections that make cats' tongues so rough, these also play no part in the physics of lapping.  Also, it seems that cats drink in a very different way to dogs, who merely use their tongues to scoop up liquid.

As well as helping us to understand how cats maintain their refined image, even  when lapping rainwater from puddles, understanding these mechanisms will inform biomechanical models and help us to design better "soft robots".


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