Gladys Puertas, age 10, of San Francisco, California, for her question:

What sort of material is a rhinoceros horn?

The rhinoceros and the reindeer have horns, so do the cow and the caribou, the goat and the giraffe and many other animals. But not all their horns are made from the same sort of material. The caribou, the reindeer and all the other deer have antler horns made of bone that grows from their foreheads. The giraffe has little fur covered horns that also are made of bone. But the horn on the nose of Mr. Rhino is not made of bone.

The horns of a cow and a goat are made of tough layers of special skin. The claws of a bird and your own nails are made of the same sort of horny material. And so is the horn of Mr. Rhino. It is made of lots of thin, horny threads, all matted and stuck together. It is hollow, but very strong and deadly sharp.