Karen Crabtree, age 8, of San Diego, Calif., for her question:

How do frogs croak?

With early spring, the frogs and toads wake up from their long winter sleep. Hop hop thry go, plop plop to the pond where they greet their friends and relatives. Then the merry froggy chorus begins. Each evening there is a concert of high notes and low notes and middle notes. It tells us that the cheerful frogs are all set to enjoy another summer.
The bullfrog has a booming voice like a big, bass drum. The little peeper frog has a high voice, thin and clear. Other frogs give forth husky croaks or raspy croaks and some croak like quacking ducks. As a rule, We only hear the frog chorus. The evening singers are hidden away in the dusky grasses and waterweeds. If you saw them, you would notice that the noisy fellows do not open their mouths to croak.
Each of the cheerful croakers keeps his big mouth shut tight in a froggy smile. From time to time, his booming note comes forth, but his mouth does not open. However, if you look closely, you see that his throat throbs with every croak. If he is the right kind of frog, he may have a balloon under his chin. This is a sac of air, and it also throbs each time the frog croaks.
Voice sounds are made by small flaps of skin called vocal cords. You have vocal cords in your throat, and so has Mr. Frog. The air coming up from your lungs makes your vocal cords tremble and vibrate. This vibration makes the sound of your voice. Mr. Frog shuts his mouth and sends air from his lungs back and forth over his vocal cords. The little flaps of skin vibrate with a croaky sound.
Mr. Croaker may be one of those frogs with a bag of loose skin under his chin. When he gets ready to croak his hush notes, this bag of skin fills up with air like a fat balloon. It acts as a sounding box and adds an extra echo to the froggy music.
Mr. Frog is the singer in the family, but Mrs. Frog may add a weak note to the chorus. As a rule she has no voice at all. But she listens. She knows that the froggy croaker is singing just for her. All those merry high notes and deep booming notes, those cheerful quacks and throaty croaks are loye songs for the ladies of the frog world.
The dog barks and the coyote howls, the cat meows and the lion roars. Monkeys chatter and the little mouse squeaks. The birds, without a doubt, are the best singers in the world. All these children of Mother Nature found their voices millions of years ago. But long, long before this, the frogs had learned to croak. The very first land animals to have voices were the ancient ancestors of the frog family.