EXPLANATORY NOTE. In the inference exercises in this book, there is a group of facts for you to explain. They can always be explained by one or more of the principles studied, like gravitation, water seeking its own level, or air pressure. If asked to explain why sucking through a straw makes soda water come up into your mouth, for instance, you should not merely say "air pressure," but should tell why you think it is air pressure that causes the liquid to rise through the straw. The answer should be something like this: "The soda water comes up into your mouth because the sucking takes the air pressure away from the top of the soda water that is in the straw. This leaves the air pressing down only on the surface of the soda water in the glass. Therefore, the air pressure pushes the soda water up into the straw and into your mouth where the pressure has been removed by sucking." Sometimes, when you have shown that you understand the principles very well, the teacher may let you take a short cut and just name the principle, but this will be done only after you have proved by a number of full answers that you thoroughly understand each principle named. Some of the following facts are accounted for by air pressure; some by water seeking its own level; others by gravitation. See if you can tell which of the three principles explains each fact: 1. Rain falls from the clouds. 2. After rain has soaked into the sides of mountains it runs underground and rises, at lower levels, in springs. 3. When there are no springs near, people raise the water from underground with suction pumps. 4. As fast as the water is pumped away from around the bottom of a pump, more water flows in to replace it. 5. After you pump water up, it flows down into your pail from the spout of the pump. 6. You can drink lemonade through a straw. 7. If a lemon seed sticks to the bottom of your straw, the straw flattens out when you suck. 8. When you pull your straw out to remove the seed, there is no hole left in the lemonade; it closes right in after the straw. 9. If you drop the seed, it falls to the floor. 10. If you tip the glass to drink the lemonade, the surface of the lemonade does not tip with the glass, but remains horizontal. SECTION 4. _Sinking and floating: Displacement._ What keeps a balloon up? What makes an iceberg float? Why does cork float on the water and why do heavier substances sink? If iron sinks, why do iron ships not sink?