I have to be honest. I have always wondered whether my dog actually got any water in his mouth. It always seemed to me that the majority of water ends up on my floor. High speed video collected and analyzed by a team of researchers from Virginia Tech (Sean Gart, Jake Socha and Sunghwan Jung) and Purdue University (Pavlos Vlachos) has shown exactly how a dog is able to drink water…and also explains why they are so messy doing it. Their research was presented at the 67th Annual Meeting of the American Physics Society Division of Fluid Dynamics (November 23, 2014 — November 25, 2014).
Unlike humans, dogs do not have complete cheeks that extend around their mouths. This means they cannot create suction, which is why they drink fluids by lapping them with their tongues. This also explains why they make such a mess! Dogs curl their tongues posteriorly while literally slamming into down onto water to displace it. Displaced water gathers on the ventral surface of their tongues which is pulled into their mouths by inertia. I was surprised to learn that water gathers at a rate approximately 5 times higher than the forces associated with gravity. After withdrawing their tongues, they close their mouths very quickly to prevent the water from falling back out.
Here is a video montage put together by the researchers: Dogs drinking water in slow motion.
American Physics Society