Ginny Franklin, age 8, of Orlando, Fla., for her question:

HOW DOES A CAT PURR?

When your cat is happy or in a contented mood, you'll hear him purring. He makes this special sound by using equipment most other animals do not have: two sets of vocal cords.

Most animals only have one set of vocal cords in their throats. In cats, one set is above the other and each set produces different sounds.

Many scientists say that the cat uses the lower cords to produce the familiar "meow" and the other ordinary sounds it makes. The cat then uses the upper vocal cords to either purr or growl.

The cat is one of the smartest of all tame animals. Along with his ability to purr, he has another unique trait: he wants to have his own way most of the time and he's stubborn enough to almost always get what he wants.