You know when you have the cap on a water bottle and you twist it up and then flick the cap until it is unscrewed and it shoots off. Well this is like that except it goes way faster.

Step 1: Drink the Water

Drink a bottle of water. Preferably an Arrowhead eco-bottle, since those are easy to crush and add pressure (if your strong enough. You will understand when you do it.)

Step 2: Fill It With Compressed Air/liquid

Open the bottle. Flip a can of compressed air upside-down and spray for about 4 seconds.
CLOSE THE CAP AS QUICKLY A POSSIBLE

PS: It should come out as a kind of liquid instead of a gas. You may not see that it is a liquid since it quickly turns into a gas and since the whole bottle will be fogged out. This is because the substance inside is not actually air, and when that substance is compressed to a certain pressure, it turns into a liquid, so companies can put more gas into the cans than they could if they used regular air. We are using the fact that it comes out as a liquid and then expands into a gas to create a large amount of pressure.

Step 3: Wait for the Expansion

Wait for about 5-10 seconds. The bottom of the bottle should pop out instead of it being in like it usually it. (Concave to Convex) See the second image for a picture of the bottom being convex.

No need to read this part if you read the P.S. on the last page
The reason for waiting is to let the liquid in the bottle to expand into a gas. It used to be a liquid while it was in the can but it is expanding in the bottle, creating a large difference in pressure between it and the standard earth's atmosphere.

Step 4: Shooting the Cap

For this step, you may want to wear earplugs or some kind of ear protection as it will make a loud bang that will make your ears ring for a minute and a half

Put your right thumb on the right side of the cap and push your thumb away from you as hard as you can to release/unscrew the cap.

The cap of the water bottle should shoot quite far from where it was shot from. If you were not holding it tight enough, it may even slip out of your hands hitting you in the gut.
You may notice some fog in the bottle after the gases are quickly decompressed. To see what I mean, look at the picture below

A simple explanation
The effects of this are similar to popping a balloon. The reason there is a loud band is the same reason why when a balloon pops there is also a bang. Since a water bottle doesn't expand as much as a balloon does, the ratio of atmospheric pressure to pressure of fluids in the water bottle are much higher than if the water bottle was a balloon.
Another way to look as how this works is think of it as a bullet. The bottle is the bullet casing, the compressed gas as the gunpowder, and the cap is the actual bullet. Pressure expands in the casing, making the bullet go flying. It is a bullet with the expansion of gases happening much slower than a regular bullet and a bullet where the actual bullet requires some convincing before it decides to fly off.

A more scientific explanation
When you made the barrier (bottle cap) to the area of lower pressure easier to overcome (unscrewing it), the pressure tried to equalize it self as fast as possible through one opening, which was the top of the bottle. Since to top of the bottle had something in front of it (the cap) that object was pushed away as it was the easiest way for the fluids to get more of itself out and equalize the pressure.

Troubleshooting for this step
If the cap did not unscrew and go flying, try doing steps 3-4 again except this time, only screw the cap on until you don't hear any hissing from the cap, and no tighter than that. This will make it much easier to flick off.