Frogs are well known to capture fast-moving prey by flicking their sticky tongues out of the mouth. This tongue projection behaviour happens extremely fast which makes frog tongues a biological high-speed adhesive system. The processes at the interface between tongue and prey, and thus the mechanism of adhesion, however, are completely unknown."
[link below]

New research (2015) has attempted to experimentally clarify the mechanisms , which appear to be a combination of viscous stickiness and microstructure adhesion.

See: Frog tongue acts as muscle-powered adhesive tape Royal Society Open Science.

Note: A similar puzzle exists for the feet-pads of many insects - which can somehow adhere to almost any surface. Some aspects of the attraction can be explained by Van der Waals forces, but others cannot.