I'll read you your riddle, if you'll read mine?

When is water not water?

When it's ice or vapor!

Right. Water is very uncertain. Most of the time it is a liquid. But if it gets too cold it turns into a solid. If it gets too hot—whisk! It is gone into the air! When it has vanished it is vapor, or water gas. Now air is a gas. The light you read by is made by burning coal gas. You cannot see a gas at all. So you cannot see vapor, or water gas. Needing a great deal more room as gas than it did as a liquid, water expands, or explodes, if confined in a kettle. This makes bubbles. We say the water boils. It will go on boiling, or expanding into bubbles, until the water all boils away, or escapes in gas, if the kettle is left over the fire.

Here is a funny thing. If you boil water over a camp fire on the sea shore, you have to heat it to two hundred and twelve degrees. But on a mountain top, water boils before it gets as hot as that. This is because, on low ground, there is more air above the water than on land a mile or two higher. The lighter the air pressure the easier it is for water to expand into gas. Therefore, it takes less heat on a high mountain to make water boil. Of course, then, boiling water away up on the Alps isn't nearly as hot as boiling water on a sea beach. In some very high places boiling water should be just about right for a warm bath, and water there would escape in gas long before it was hot enough to boil an egg.