Why you can hold a paper cup over a flame without burning it

Paper burns. Especially if it has some incriminating evidence on it and it's in a dramatic film about corrupt lawyers. But even in everyday life, paper reaches a certain temperature, and bursts into flame. And yet sometimes it doesn't.

There is a very simple thing that you can do to a paper cup to keep it from catching fire even if you take a blowtorch to it. This weekend, amaze your BBQ pals with this fun, fire-based experiment.

First, fill the cup with water. The water doesn't have to coat the outside, and you don't have to expose only the wet parts of the cup to heat. Just fill a paper cup with water and start trying to burn it from the outside. The water will boil vigorously inside the cup, but the paper itself will not combust.

It's tempting to say that the water keeps the paper from reacting with oxygen, which is needed for the oxidization process of fire. But the paper doesn't lack oxygen, it just lacks heat. Although the fire is scorching it from one side, the water is cooling it from the other. It can't get hot enough to burst into flame.

Paper burns at around 500 degrees Fahrenheit (although book paper burns at a relatively balmy and famous 451). Water boils at a mere 212 degrees. When the flames heat the paper to above 212, the water nearby turns to vapor steams away, leaving cooler water to take its place. The boiling water can't get above 212 degrees, and the paper is thin enough that this water keeps its temperature from climbing high enough to combust. It's only when the water on the other side of the paper entirely boils away that the paper cup will burn away.

Via Factoids.