Under Pressure

First, check this out - it's a short gif with a longer explanation :)

So what's going on here? One part you probably understand: the candle burns up all of the oxygen (fire feeds on oxygen, can't exist without it) available in the glass and the candle goes out.  But why did all of that red liquid defy gravity and move up the glass next to the candle?

I get very upset every time I hear the starting beat of this song and it's actually Ice, Ice, Baby

For that, we need to discuss pressure - and I don't mean the kind you're under during state testing! We're all under this pressure all the time: the pressure of the atmosphere pushing down on us, just like if we were at the bottom of the ocean and all of that water was pushing down on us. It might seem weird that air can push down on us, but that's only because we can't see it - there's MILES of air above us, so even though each particle doesn't weigh much, it adds up.

 

It's one square inch because we measure pressure in pounds per square inch - PSI! Your bike tire, basketball, and inner tube all have a recommended maximum PSI, and if you went over that there might be too much pressure inside and it would EXPLODE :) (source)

What if you were inside a building though? Wouldn't the roof protect you? Well...no, because very few buildings are totally sealed, and something funny happens when there's areas of different pressures: air from the higher pressure moves in to the lower pressure area until they're equalized. Air always does this! That's why if you keep inflating (or squeezing) a balloon enough, the higher pressure inside will escape to the lower pressure outside, even if it has to go through the side of the balloon:

Too much pressure

This is also how we breathe! When you breathe out, your lungs are collapsing (hopefully not all the way), making them smaller, and the now higher pressure inside will cause the air to rush out. When you breathe in, your lungs expand, which lowers the pressure inside of them because there's more space; then air moves in from the now higher pressure outside.

 

More volume with the same amount of stuff will always have lower pressure (source)

Let's get back to the candle situation.  What happens to the pressure inside the glass when all that oxygen burns up? It lowers! Then the now-higher pressure outside tries to get into that lower pressure, but it has to go through the liquid - so it pushes the liquid up inside the glass, trying to equalize. This is exactly how a straw works as well!

Here's another awesome example of pressure, where an oil tank that was almost empty caught a spark and the oil burned up - not enough to explode, but enough to eat all the air inside the tank. The higher pressure of the air had to get in somehow...

This is real.