From the brains behind brains on, this is the moment of um.

Answering those questions that make you go, um, um, um, moment of um comes to you from APM studios. I'm Anna Goldfield.

You look forward to your birthday for a whole year. You wake up, see your friends at school,

get home, open presents, and a cake just for you. As your family

gathers around the table, they tell you to make a wish and blow out the candles. You take a big

breath in and, oh no, your brain is frozen because you can't stop asking yourself this question.

My name is Alex, and I'm from Westwood, New Jersey. My question is, where does a candle go when it burns?

So there is kind of a long process that is happening in the flame to go from these big molecule

that is a wax and at the end you get water and CO2. I'm Fabian Goude.

I'm a scientist studying combustion chemistry at the University of West Virginia.

So a combustion is a chemical reaction between a fuel plus oxygen to give carbon dioxide plus water and heat. So most of the fuel we are using all based on carbon.

Wax as carbon and hydrogen, which is very similar to other fuels that we use for our cars.

So wax is a solid at room temperature. The first thing that's going to happen is that candle wax is going to melt

at the top of the candle and you can see it. And then it's going to go up the week and then start to

be vaporized in the gas. Once it's in the gas at very high temperature is going to break down.

So at the bottom of the flame, you are going to see this very pretty blue color that is going to be emitted by the molecule that are

product from the wax being broken down in smaller molecules. That's not the end because these smaller molecules are going to continue reacting and

a little bit higher in the flame.

You are going to start adding these orange and yellow.

And what's happening here is that all the small molecules that used to be the wax now are reacting to form

what we call particles. So there are small balls of carbon so small that you cannot see them

with your eyes. And then as you go up in the flame, these are going to react even more

...