But we might also compare Mother Nature's way of changing rocks to the cooking that goes on in our kitchens. She uses not only heat, but water and other things, including salt and soda. Both the salt and some of the water in the rocks comes from--you'd hardly guess it--the seas! Not the seas of to-day, but the seas of yesterday, when these rocks were made. Then the pores were filled with water and the water has been kept shut in down there by the rocks above ever since. From this sea water comes the salt. The salt in the water, when heated, helps to dissolve the rocks so that the different materials in them can separate and come together again in new ways, and so form new rocks. You know when you go to the lavatory to change your hands from dark to light what a lot of difference it makes whether the water is hot or cold and whether you use soap. The soap helps dissolve the dirt on your hands just as the salt helps dissolve the rocks. The soda which Nature also uses is particularly good for dissolving rock that will hardly dissolve without it; silica, for instance, out of which are made the hardest of the sand grains, the sand in sandstone, the sharp, glassy edges of grass blades, and the blades of wheat, and the stalks of corn. Whenever there is a great deal of silica in rock you find soda mixed right with it. This, having the rocks already salted and mixed with soda before putting them in the oven, Mother Nature has always found _so_ convenient! ONE PEBBLE MAY PLAY MANY PARTS I, in my time, may have been many kinds of rock. First, heaved up out of the sea by the earliest wrinkling of the cooling earth as granite; then weathered away into soil and carried by rivers to the sea, where I was remade the first time, maybe, as part of the "dough" in a pudding stone; then up again in an earth wrinkle and again back to sea, this time to be made into some one of the clay stones, and then back to granite again. Anyhow here I am, a little freckled granite pebble talking myself red in the face because I've got so much to say, such wonderful things to tell, and only a few hundred pages to tell it in! II. How Do They Know? But, after all, how do they know that one rock changes into another? No one ever caught a rock doing this, did they? Not quite, but almost. To explain, I must first tell you about the fossils that are found in stone. Haven't you often noticed in marble curious figures that reminded you of sea-shells? They were sea-shells but have been turned to stone, and things similarly changed while still keeping their original form are called "fossils."