And a curious thing about those which build of light pieces of bark or reed is this, that they will make the top-piece come over so as to hide their heads, and prevent you from seeing them. Some build of sand; and then as the house would be so light that the water running from the spring might wash it down and carry it away, the wonderful little creature takes care to anchor it by fastening a pretty large stone to it when it has nearly finished it. And as the worm anchors it when it is too light, so it lightens it when it is too heavy, by fixing a bit of light wood or hollow straw to it to buoy it up." [Illustration: Sand Nest, balanced with a Stone.] [Illustration: Nest of Caddis-worm, balanced with Straws.] "Well, this is truly a wonderful insect. Uncle Philip." "Truly so indeed, boys. In all these cases it uses its water-proof cement, and if you break its house to pieces, and will patiently watch, you may see it build another. The insect always lives with its head out of doors, and its body inside; so that its head is firm and hard, while its body is soft." "Uncle Philip," said one of the larger boys, "there is one thing I have been thinking about, as you have been talking: these little masons have no trowel, but I believe I know of one animal that uses something like that tool." "Ah! What animal is it?" "Why, I was reading the other day something about the beavers building their dams and their houses, and the book said that they built their houses of logs first, and then plastered them with mud, and that they used their tails for trowels." "I am very glad to find that you remember what you read; but I am sorry that your book did not tell you the truth. There have been very strange stories told about the beaver; and these stories have been taken from one book and printed in another, so that an untrue account has gone down for a great many years. The beaver is very ingenious, but is not quite so much of a mason as you suppose." "Well, Uncle Philip, will you tell us the truth about it?" "Yes, boys, I will, so far as I know it myself. I have seen these animals, for they were once a great deal more common in our country than they are now; and many of the stories told of them are not true. But before I begin, let me tell you of one book which I think does tell the plain truth about them; and the truth is curious enough." "What book is it, Uncle Philip?" "It is a book written on American Natural History, by Doctor John Godman. I knew him, boys, and a most excellent man he was. He is now dead--and he died a Christian. The book he wrote you will find worth reading, when you get old enough to understand it. But now for the beaver.