Mr. Beaver's chief purpose in building these dams seems to be to keep his front-door yard full of water. This may look like a funny idea at first, but in this, as in other things, Mr. Beaver shows he has a very wise head on his shoulders; for one peculiarity of his life is that he is obliged to come and go through the cellar door. As he doesn't want any of his enemies--the wolf, the coyote, and all that class of people--to use this door, he keeps it under water. And in winter-time, when he goes out to the wood-pile to get something to eat, the water must be deep enough so that the pond doesn't freeze solid to the bottom. [Illustration: A BEAVER HOME WITH TIME LOCK Here, as it looks after being made secure against hungry wolves and the Winter winds.] As for those professional highwaymen, the wolves and coyotes, that are so much bigger than he is, Mr. Beaver keeps out of their way in Summer, when they don't bother much about him, anyway, as he sticks so close to the water and is hard to catch. In the Winter, when they get hungry and desperate and would break into his house, if they could, he makes it practically burglar-proof, by putting on a time lock; a lock that just won't open, even to a wolf's sharp claws, until Spring. And in the simplest way. Just before Winter sets in Mr. Beaver plasters the outside of his house with mud, and the mud freezes as hard as a stone. But sometimes, even among the beavers, there are shiftless characters, like that Arkansas man who just _wouldn't_ look after his roof. These careless beavers don't plaster their roofs. But then, just see what happens! Some hungry wolf comes along and breaks through and has a nice fat beaver for supper, maybe. And maybe not; for, even in that case, if Mr. Beaver wakes up in time, he dives down through the cellar door and into the tunnel and out under the ice. "Aha! You got fooled that time, didn't you? You mean old thing!" (Can't you almost hear him say it?) In putting the mud coating on their houses or dams the beavers carry it in their fore paws. Sometimes, in a very steep place, they climb up the roof with three feet and hold the mud with one. When they have delivered the mud they use these same little paws to pat it down--not their trowel-like tails, as one would naturally suppose. THAT MYSTERY ABOUT THE BEAVER'S TAIL