Liquids, however, while they are inclined to be constantly on the move, can only go in one direction--down hill, and they go down fast or slow if there is a chance, in proportion to the amount of stick-together properties they have. Liquids can never go up of their own accord, excepting in the process of evaporation, and then only when changed into gases. A lake of water will dry up completely by evaporation unless fed by streams of water constantly flowing in, because evaporation is constantly taking place wherever water is exposed to the air. What Makes the Water Boil? What we call boiling in the water we see when water is put over a hot fire long enough to make it boil, is the changing of the water from what we generally regard it--a liquid--into gases. Water consists of two gases--hydrogen and oxygen--in fact, two parts of hydrogen gas and one part of oxygen gas when mixed will always make pure water. Now, then, if liquid water is heated to a certain point or temperature it turns into the two gases, oxygen and hydrogen, and comes to the top of the water, which still remains in liquid form, in the form of a bubble and explodes into the air--not a very loud explosion, but still an explosion. The process of turning liquid water into gases is a gradual one, and that is why the water does not all turn into one large bubble at once and explode away. If you keep the fire going long enough, all the water in the vessel will explode away into the air, a few bubbles at a time. If you hold a cold plate over the vessel as the bubble explodes you can catch some of these gases in the form of bubbles on the under side of the plate, which are again liquid water. When the water becomes hot enough it turns into bubbles and as bubbles rise that is what makes the boiling you see. When the same gases then come together again in a certain proportion under proper temperature they turn into liquid water. At What Point of Heat Does Water Boil? The boiling point of water is the temperature at which it begins to pass into the form of gases. This varies in different altitudes. At the sea level the boiling point is at 212° Fahrenheit. On the top of mountains, for instance, water would boil at a much lower temperature. It would be possible to go high enough in a balloon so that the water would fly from the pan in the form of gas without making the water hot. Also, a mile below the level of the sea it would take many more degrees of heat to make the water boil. It is said that high up in a balloon you could not boil an egg hard in a pan of boiling water if you kept it in the boiling water for an hour or more, whereas we know that an egg will be hard-boiled if we keep it in boiling water down where we live for more than five minutes.