Source : Imponderables by David Feldman
Why do frogs close their eyes when swallowing?
There is a downside to those big, beautiful frog eyes. While they may attract the admiration of their beady-eyed human counterparts, frog eyes bulge not only on the outside but on the inside of their face. The underside of their eyeball is covered by a sheet of tissue and protrudes into the mouth cavity. frogs literally cannot swallow unless the use their eyes to push the food down their stomach. Richard Landesman, zoologist at the University of Vermont, amplifies:
"In order for frogs to swallow, they must be able to push material in the mouth backwards into the oesophagus. Humans use their tongue to accomplish this task; however, frogs use their eyes. By depressing their eyes, food can be pushed posteriorly in the mouth. Frogs also use this same mechanism to breathe, since they lack a diaphragm."
If we ate what frogs eat, we might close our eyes when swallowing too.