Lapping: How Cats Drink
How cats drink is a surprisingly complicated process, and it wasn't until researchers at MIT published a paper in 2010 that it was properly understood.
Vertebrates with complete (closed) cheeks like pigs, sheep, horses, and people use suction to drink. Once drawing the liquid into their mouths, they push it to the back of their mouth with their tongues. Vertebrates without complete cheeks like cats, dogs, and most carnivores, cannot generate suction after weaning. They gave that ability up in order to open their mouths wider. Instead, they lap.
Dogs lap by making "scoops" out of their tongues and scooping the liquid up into their mouths. Cats do not. Instead, cats "flick" the surface of the liquid with the flat, top side of the tip of their tongues. Their tongues, which are mostly covered with papillae, tiny, curved spines made out of keratin (the same stuff that fingernails are made out of), are smooth just at the tip for this purpose. Liquid adheres to their tongues and is drawn up towards the cat's mouth by fluid inertia.
To drink the liquid, cats have to bite the top of the liquid column when it is at its thickest. This is the "pinch-off" moment when the force of inertia pulling the liquid up is perfectly balanced by the force of gravity pulling the liquid down. Too soon and the liquid is still on its way up. Too late and the liquid has started to fall back down.
Lapping is common across all feline species with the cat's mass determining how frequently they lap. They are inversely proportional, so big cats like lions and tigers with larger tongues lap more slowly than common housecats. The tongue of a housecat has a maximum speed of 78 +/- cm/s and they lap 3.5 +/- 0.4 times per second. The vertical displacement, how high they must draw up the water, is 3.0 cm. The radius of the surface of the cat's tongue that touches the water is only 5 mm.
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For a closer look at how cats lap and how their tongues work, check out this episode of PBS's Deep Look series.