12. If you put the cup in sidewise, it sinks. 13. The water in the cup, when lying on its side, is exactly as high as the water in the dish pan. 14. If you put a glass into the water, mouth first, the water cannot get up into the glass; if you tip it a little, there are bubbles in the water and some water enters the glass. 15. If you let a dish slip while you are wiping it, it crashes to the floor. 16. It is much harder to hold a large platter while you are wiping it than it is to hold a small butter plate. 17. If you set a hot glass upside down on the oilcloth table cover, the oilcloth bulges up into it when the hot air and steam shrink and leave a partial vacuum within the glass. 18. If you spill any of the dishwater on the floor, it flattens out. 19. You may use a kind of soap that is full of invisible little air bubbles; if you do, the soap will float on top of the water. 20. When you drop a dry dishcloth into water, it floats until all the pores are filled with water; then it sinks. SECTION 5. _How things are kept from toppling over: Stability._ Why is it harder to keep your balance on stilts than on your feet? Why does a rowboat tip over more easily if you stand up in it? In Pisa, Italy, there is a beautiful marble bell tower which leans over as if it were just about to fall to the ground. Yet it has stood in this position for hundreds of years and has never given a sign of toppling. The foundations on which it rested sank down into the ground on one side while the tower was being built (it took over 200 years to build it), and this made it tip. But the men who were building it evidently felt sure that it would not fall over in spite of its tipping. They knew the law of stability. [Illustration: FIG. 13. The Leaning Tower of Pisa.] All architects and engineers and builders have to take this law into consideration or the structures they put up would topple over. And your body learned the law when you were a little over a year old, or you never could have walked. It is worth while for your brain to know it, too, because it is a very practical law that you can use in your everyday life. If you wish to understand why the Leaning Tower of Pisa does not fall over, why it is hard to walk on stilts, why a boat tips when a person stands up in it, why blocks fall when you build too high with them, and how to keep things from tipping over, do the following experiment and read the explanation that follows it: