Today’s Wonder of the Day was inspired by Natalie from , . Natalie Wonders, “why does water bubble while it boils” Thanks for WONDERing with us, Natalie!

We were walking through the Wonderopolis pasture the other day when we overheard an interesting conversation between a cow and a horse:

Cow: I'm so hungry I could eat a horse.

Horse: Hey! That's not funny.

Cow: Sorry. Just kidding! Maybe we should head to the barn and whip up some grub.

Horse: What are we going to whip up? We don't know how to cook. We can't even boil water!

At one time or another, you may have heard someone say that they're such a bad cook that they can't even boil water. You would indeed have to be a really bad cook not to be able to boil water.

After all, when it comes to boiling water, all you need is a pot full of water and some heat. With enough heat and time, that placid pot of water will turn into a bubbling, boiling cauldron full of hot, steamy water.

Speaking of bubbling, what exactly are those bubbles you see in a pot of boiling water? Some people believe it's air, since many bubbles you may be familiar with, such as soap bubbles, are indeed filled with air. Others believe it's hydrogen or oxygen escaping as a result of a chemical change in the nature of water when it boils.

Neither of these is true, though. When you first pour water into a pan and begin to heat it, you'll notice bubbles along the walls of the pan. These bubbles are indeed air. Most water has some air dissolved in it. As you begin to heat the water, this dissolved air escapes the water. These bubbles aren't the bubbles associated with boiling water, though.

When water is boiled, it undergoes a physical change, not a chemical change. The molecules of water don't break apart into hydrogen and oxygen. Instead, the bonds between molecules of water break, allowing them to change physically from a liquid to a gas.

You probably already know that water comes in three forms: solid, liquid, and gas. The solid form we know as ice. The liquid form is, of course, water. The gaseous form is water vapor. Water vapor exists around us in the air nearly all the time. We just can't see it.

To convert a liquid to a gas via boiling, the liquid must be heated until its vapor pressure equals the atmospheric pressure. For water, this occurs at approximately 212º F (100º C). That's why 212º F (100º C) is considered the boiling point of water. In reality, however, the boiling point of water can be higher or lower depending upon many different factors, such as altitude, atmospheric pressure, and other chemicals present in the water, to name a few.

When water is boiled, the heat energy is transferred to the molecules of water, which begin to move more quickly. Eventually, the molecules have too much energy to stay connected as a liquid. When this occurs, they form gaseous molecules of water vapor, which float to the surface as bubbles and travel into the air.

Instead of air, the bubbles in a boiling pot of water are actually made up of water — it's just water in its gaseous state! What looks like a pot full of water and air is really just a pot full of water in two different physical states.

Wonder What's Next?

Tomorrow’s Wonder of the Day is sure to stop you in your tracks!