When boiling water, where does the air that fills up the never-ending bubbles come from?

Aha! How do you know it's air that fills these bubbles? Isn't there a cloud of steam coming from your boiling pot of water, with the bursting bubbles at the surface? Doesn't the water level go down in your pot if you leave it boiling for a long time? See where I am going?

The answer to the question is that it is not air that fills the water bubbles, but water! Water vapor to be exact. Water in the form of gas. Water turns to vapor when it is heated. The vapor is trapped in bubbles that form at the bottom of the pot. The bubbles filled with vapor rise to the surface of the pot, where they burst, releasing the water vapor into the air above the pot. There, the water vapor cools and turns into the steam you see coming off the pot.

The steam dissipates into your kitchen (sometimes ending up as water condensation on cold surfaces), and that much water is lost to the pot. Over time, the water level in the pot goes down.

How do parachutes make it safer to fall?

In my last post, I talked about terminal velocity and how it was different according to the situation. Ducklings have a smaller terminal velocity than humans because they are a lot lighter. Another way to reduce the terminal velocity is to increase the cross sectional area of the falling object. You can think of cross sectional area as what you get if you light up the object from straight above and look at the shadow on the ground. The shadow of a parachute is a lot larger than the shadow of a human. The large cross sectional area of the parachute reduces the terminal velocity of the human attached to it and makes for a much gentler landing.