All these things would really happen, and many more changes besides, if water did not evaporate. Yet the evaporation of water is a very simple occurrence. As the molecules of any liquid bounce around, some get hit harder than others. These are shot off from the rest up into the air, and get too far away to be drawn back by the pull of the molecules behind. This shooting away of some of the molecules is evaporation. And since it takes heat to send these molecules flying off, the liquid that is left behind is colder because of the evaporation. That is why you are always cold after you leave the bathtub until you are dry. The water that evaporates from your body uses up a good deal of your heat. [Illustration: FIG. 49. Diagram illustrating how in the evaporation of water some of the molecules shoot off into the air.] Gasoline evaporates more quickly than water. That is why your hands become so cold when you get them wet with gasoline. Since heat is required to evaporate a liquid, the quickest way to dry anything is to warm it. That is why you hang clothes in the sun or by the stove to dry. Try these experiments: EXPERIMENT 33. Read a thermometer that has been exposed to the room air. Now dip it in water that is warmer than the air, taking it out again at once. Watch the mercury. Does the thermometer register a higher or a lower temperature than it did at the beginning? What is taking up the heat from the mercury? EXPERIMENT 34. Put a few drops of water in each of two evaporating dishes. Leave one cold; warm the other over the burner, but do not heat it to boiling. Which evaporates more quickly? WHY THE SEA IS SALT. You remember various fairy stories about why the sea is salt. For a long time the saltness of the sea puzzled people. But the explanation is simple. As the water from the rains seeps through the soil and rocks, it dissolves the salt in them and continually carries some of it into the rivers. So the waters of the rivers always carry a very little salt with them out to sea. The water in the ocean evaporates and leaves the salt behind. For millions of years this has been going on. So the rivers and lakes, which have only a little salt in them, keep adding their small amounts to the sea, and once in the sea the salt never can get out. The oceans never get any fuller of water, because water only flows into the ocean as fast as it evaporates from the ocean. Yet more salt goes into the ocean all the time, washed down by thousands of streams and rivers. So little by little the ocean has been growing more and more salty since the world began. [Illustration: FIG. 50. A view of the Dead Sea.]