When under water, a frog takes in oxygen through its skin.  When a frog dives under water, he does not come to the  surface as quickly as you have to when you dive.  Why not?

Frog swimming underwater

The frog, of course, cannot breathe under water, as it did when it was a tadpole.  When a frog is on the bank or pond, it breathes with its lungs, which are somewhat like your lungs.

There is always some air mixed with water in the pond.  When the frog is under water, it can take a little of the air it needs through its skin.

This explains why the frog can stay under water all winter.

If you live where the winters are cold, you’ve probably noticed that the frogs seem to disappear when cold weather comes.

Many frogs dive into ponds and bury themselves in the muddy bottom, and quickly fall asleep.  Sleeping all winter is called “hibernation.”

While the frog is sleeping, its body keeps so still that it can get along without any fresh air until it wakes in the spring.  During its winter sleep, the frog lives on the food stored in its fat body.  – Dick Rogers