And the dust she works with must be as fine as powder and as dry as a bone. Then it absorbs the saliva, and when it dries it is almost like stone. In fact it's a kind of cement, like that men use for sidewalks and for buildings and bridges. [Illustration: _Copyright by Brown Brothers._ FABRE STUDYING THE MASON-BEE] But this wonderful old teacher and his boys[16] found that even this isn't all this little house-builder and house-keeper has to think of. She must have dust that is really ground-up stone! So she digs in the roadway where the bits of stone in this stony soil have been ground to powder and then packed hard by the wheels of the farmer's cart and by the hoofs of horses and oxen drawing their heavy loads. But what did Mrs. M. B. do for ground-up stone in the long ages before man came along with his carts? Mr. Earl Reed, who, beside being the distinguished etcher of "The Dunes," is a close observer of nature in general, tells me he has often seen a mason-bee gathering the pulverized stone at the base of cliffs. Evidently the mills of the wind and rain, that we have read of in previous chapters, had Mrs. B's wants in mind too. [16] The boys were a great help. You ought to see what Fabre himself says about them in that famous book of his. BEING A MASON-BEE FOR A LITTLE WHILE Now, just to show you one more thing about Mrs. Mason-Bee as a house-builder--how clever she is--let's try something right here. Let's suppose ourselves--yourself and myself--Mrs. Mason-Bees. We have got a home to build for some baby mason-bees that will be along by and by. Say we already know that we must use this stone dust of the roadway, and that we must make our mortar not with _water_ but with _saliva_. Here's the _next_ problem: Shall the mixing be done where the building is going up over there? That's the way human masons do it. But Mrs. Mason-Bee evidently thinks otherwise, for at the very time she is prying up those atoms of dust with so much energy, you notice she is doing her mixing. She rolls and kneads her mortar until she has it in the shape of a ball as big as she can possibly carry. Then "buz-z-z-z!" Away she goes, straight as an arrow, back home, and the mortar is spread where it is needed. You see, after all, this is the best way. If she didn't turn the dust into mortar before she started, so a good-sized lump of it would stick together, she couldn't carry much of it at a time, and it would be forever and a day before she could get her house built. As it is, the pellets she carries are of the size of small shot; a pretty big load, let me tell you, for a little body no bigger than Mrs. Mason-Bee. And remember, this goes on all day long from sunrise to sunset. Without a moment's rest, she adds her pellets to the growing walls and then back she goes to the precise spot where she has found the building material that best suits her needs.