Jerome Chisom, age 11, of San Diego, Calif., for his question

HOW LONG CAN A BEAVER HOLD HIS BREATH UNDERWATER?

A few weeks ago, a busy beaver appeared in Andy's column to explain why he builds a dam. But one good question always leads to another. So he reappears for a return engagement to explain how long he can remain submerged as he commutes to and from his building projects.

The remarkable beaver, as we know, is an air breathing mammal who takes to the water to protect himself from his more frisky foes. Naturally he must come up for air and therefore can remain submerged only as long as he can hold his breath. Experts in animal behavior report    that he can stay below as long as 15 minutes.

Usually it takes but a couple of minutes to swim from the bank of his private    pool to his island home. Often he makes this    trip with his nose,    eyes and ears above the water level.    After all, no hungry bobcat is likely to pursue him into the water.

Some people claim that a beaver uses his flat scaly tail to pat the pieces into his sturdy dam. Not so. His tail is a rudder to steer him when he swims and dives, though sometimes he flicks it to gain underwater speed. He also uses it to prop himself up when he fells trees. Sometimes he smacks it on the surface with a splash that resounds a mile through the woods. This is an alarm signal to warn his relatives that an enemy is prowling the neighborhood.