Because any bulk of cork is lighter than an equal bulk of water; a cup filled with cork will weigh less than when it is filled with water. When lead is put in water, why does it sink to the bottom? Because the lead is heavier than an equal bulk of water. When cork is put in water, why does it rise to the surface? Because the cork is lighter than an equal bulk of water. Does heat increase the weight of any substance? Heat does not increase the weight of any substance. How do we know this? Because a piece of iron when cold will weigh as much as when heated red-hot. When a liquid is heated, does it become lighter or heavier? It becomes lighter, bulk for bulk, when heated. Why does a liquid become lighter when heated? Because the same liquid is expanded and takes up more room, although it has no more weight. Why will a gallon of cold water weigh more than a gallon of hot water? Because a gallon of cold water, when heated, will make more than a gallon of hot water. When do we get the most molasses, by buying it in hot or in cold weather? We get the most by buying it in cold weather, because a gallon bought then will make more than a gallon when the weather becomes warmer. Does heat expand the air and make it lighter? Heat does expand the air and make it lighter. What does the air do when heated? It ascends or rises up, because it is lighter, and the cold or heavier air falls to take its place. What part of a room, in which there is fire, is the warmest? The part next to the ceiling is the warmest, because the heated air always ascends. Why does a soap-bubble ascend? Because it is filled with heated air from the lungs, and is, therefore, lighter than the air around it. Why do balloons ascend? Because they are filled with a gas lighter than the air around them. Why does a chestnut split open when roasting? Because the air in it is expanded by the heat, so that it bursts the shell. Why does the chestnut not split open when a hole is made in the shell? Because the air can then escape as it expands, without bursting the shell. What becomes of the air in an apple, when the apple is roasted? The air, upon being heated, bursts through the peel of the apple, carrying the juice with it. Why does an apple become soft when roasted? Because the air in the cells of the apple expands, and breaks those cells, so that the apple becomes soft. What part of the apple first becomes soft? The outside, because it is first heated. Why do all fruits and vegetables become soft when cooked? Because the heat expands the air in them, and thus breaks up their cells, so that they become soft. Why does wood make a snapping noise when burning? Because the air in the cells of the wood bursts them apart, thus making the noise. Why are sparks of fire thrown out? Because the heated air bursts the cells with such force as to throw off small pieces of the burning wood. What kinds of wood snap the most?