What Makes a Cold Glass Crack if We Put Hot Water Into It? Hot water will not always cause a cold glass to crack, but is very apt to, especially a thick glass. The very thin glasses will not crack. The test tubes used by chemists are made of very thin glass, and will not crack when hot liquids are poured into them. When a glass cracks after you have poured a hot liquid into it, it does so because, as soon as the hot liquid is put in, the particles of glass which form the inside of the glass become heated and expand. They begin to do this before the particles which form the outside of the glass become heated, and in their efforts to expand the inside particles of glass literally break away from the particles which form the outside, causing the crack. The same thing happens if you put cold water into a hot glass, excepting in this instance the inside particles of the glass contract before the particles which form the outside of the glass have had time to become cool and do likewise. What Causes the Gurgle When I Pour Water from a Bottle? The air trying to get in causes the gurgle. Air has one strong characteristic which stands out above everything else. It wants to go some place else all the time. When it learns of a place where there is no air it wants to go there above all things, and goes at it with a rush. Now, when you turn a bottle full of water upside down, the water comes out if the cork is out, of course, and as soon as the water starts out the air strives to get in, and every time you hear a gurgle you know the air is getting in. Every gurgle is a battle between the water and the air. Sometimes the air comes and pushes the water back enough to let it slide into the bottle; sometimes the water pushes the air back, and thus they fight back and forth. The water always gets out and the air always gets in. In doing so they make the gurgle. Where Does the Part of a Stocking Go That Was Where the Hole Comes? Perhaps this is a foolish question, but many boys and girls have been puzzled for an answer to it. When you put your stockings on they have no holes in the feet, and at night, when you take them off, there are often quite large holes in them. The answer is the same as in the case of the lead in the lead-pencil. The lead in the pencil wears away. You can see it wear away because that is what makes the marks.