Most of us who have pets have, at one time or another, watched them lap up water or some form of liquid from a bowl. Cats and dogs, unlike us, do not have complete cheeks, therefore they cannot form suction to draw the water out of the bowl the way we can, for instance with a straw. It is also why they cannot give us a big smooch and we have to accept a sloppy swipe of the tongue as a ‘kiss’. In order to drink, we have assumed that they use their tongues much like a soup ladle and scoop water from the bowl. Judging from the mess of water constantly spread around our dog’s bowl, this is certainly true, at least for dogs.
But, recently, a scientist from MIT (in Cambridge, Mass), sat at breakfast watching his own cat drink from her bowl. And, he was intrigued. Subsequently, he recruited a few colleagues and, on their own time (not funded by university time or funds), they filmed the cat, and later nine others, drinking. The high-speed video revealed that cats do not make ladles with their tongues.
Instead, they employ a tricky combination of physics principles, inertia and surface tension pitted against gravity.
Cats simply dip their tongue into the water and pull it back up. The water tends to adhere to the tongue and be drawn upward into the mouth because 1) the tongue has imparted some upward motion onto the water, called inertia, and 2) because water molecules like to stick together. The ‘stickyness’ of water is due to molecular forces holding the atoms together. Water likes to adhere to itself, a simple phenomenon we know as ‘surface tension’. We see surface tension all around us. For example, when water beads up and runs down our windshields, it is the surface tension forming the beads. The beads readily contact and merge with other beads, forming larger droplets.
In the cat’s case, gravity will eventually win out, and impart a force larger than the forces maintained by surface tension. And, the water will fall back into the bowl. But, hopefully not before some of the water has made it into the cat’s mouth along with the tongue. A careful trick of timing maximizes the amount of water that makes it into kitty’s mouth.
A trip to the local zoo revealed that all felines may in fact use this unique physics trickery to get a drink. Lions and tigers both showed this same behavior. Just goes to show you, even our household pets have something new to teach us if we pay attention!