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Frogs’ remarkable power to tongue-grab prey — some as big as mice or as oddly shaped as tarantulas — stems from a combo of peculiar saliva and a supersquishy tongue.
The first detailed analysis of the stickiness of frog saliva shows that the fluid can shift rather abruptly from gooey to runny, says mechanical engineer Alexis Noel of Georgia Tech in Atlanta.

[...] Noel and colleagues found that this saliva is what’s called a shear-thinning liquid, which grows thinner and easier to stir or smear around when force is applied. Smacking into a fly jolts saliva from its sticky phase — more viscous than honey, she says — into the more “liquidy” phase “flowing into all the small cracks” of the insect body. As the tongue returns to the mouth, the spit thickens again, intensifying the grip.

[...] But once the fly is in the mouth, the tongue’s grip needs to loosen so the fly can slide down the gullet. “Frogs actually use their eyeballs while swallowing,” Noel says. Eyeballs sink from bulges to barely bumps, dipping inside the head and pushing food back toward the throat. The eyes’ impact jars the saliva into a runnier phase, easing its grip on the prey.