But the greatest of bee workers in the soil is the mason-bee. You can get an idea of what a useful citizen the mason-bee is when I tell you that one of the little villages of one species sometimes contains enough clay to make a good load for a team of oxen. Yet for all that, they might have gone on with their work for years and years to come--just as they have for ages in the past--and people wouldn't have thought much about it, if it hadn't been for some boys. One time, in a village in southern France, a school-teacher, who was getting on in years, took his small class of farmer boys outdoors to study surveying--setting up stakes and things, you know, the way George Washington used to do. It's a stony, barren land--this part of France--and the fields are covered with pebbles. The teacher noticed that often when he sent a boy to plant a stake, he would stoop every once in a while, pick up a pebble and _stick a straw into it_! That's what it looked like! Then he would suck the straw. Well, to make a long story short,[15] these pebbles had on them the little clay cells of the mason-bee. Mrs. Mason-Bee fills these cells with honey, lays an egg in the honey, and when the babies come along--don't you see? In other words, Mother Bee not only puts up their lunch for them, but puts them right into the lunch! This makes it convenient all around; for, like almost all insect mothers, Mrs. Mason-Bee is never there after the babies come. [15] The whole story is told in the famous book, "The Mason Bee," by Henri Fabre. He was the teacher. [Illustration: MASON-BEE CELLS AMONG THE ROCKS] There were so many of these pebbles scattered over the plain, and the bees that were building new homes or repairing old ones flew so straight and so fast between the pebbles and a near-by road that "they looked like trails of smoke," as Fabre expresses it. Now, you may well wonder why the bees flew clear over to that road to get dirt to build their nests when there was plenty of loose earth right at their own door-steps; right around the pebbles themselves. Isn't that queer? Well, here's something that sounds stranger still. Mrs. Mason-Bee takes those extra trips because a roadway is so much harder to dig in! It's not because she needs the exercise, goodness knows--this busy Mrs. Mason-Bee--but because the hard earth of the roadway makes the strongest homes; that is, when she finally gets it dug out and worked up. And here's another thing that will seem odd at first; although the soil she thus works over must be dampened before she can plaster it into the walls of her home, she just won't use damp soil to begin with. Nothing will do her but dust, and dust that she herself scrapes from the roadway. The reason of this is that the moisture already in the soil will not answer at all. She has got to knead the soil carefully and thoroughly with saliva, which acts as a kind of mortar. This saliva, of course, she supplies.