Done right, this experiment has the most rewarding whoosh. It's from the Nat Geo Kids book Try This!
You’re mimicking the part of Earth’s water cycle in which evaporated water (water vapor) cools and condenses, forming clouds as they connect with dust. In this experiment, the alcohol acts like the dust, providing something cool for water droplets to attach to.
When you pressurize the soda bottle by pumping air in, the air molecules collide with each other and warm the bottle. Releasing the pressure causes the water vapor to condense quickly, forming a cloud.
Fit your cork to your bottle. You may need to shave the sides off the cork to get a tight fit. Your aim here is the tightest possible seal. Then trim the cork to the length of the bike pump needle. You want the needle to be able to pass through the cork into the bottle, maintaining a tight seal.
Make a path for the cork, using your skewer, drill bit, or needle. Be sure the object you use to pierce the cork is narrower than the bike pump needle, or you won’t have a tight seal. When the cork is ready, take it out of the bottle.
Pour one teaspoon of rubbing alcohol into the bottle and screw the cap back on.
With the cap on, turn the bottle horizontal and roll it so that the alcohol sloshes around and coats the inside of the bottle thoroughly and evenly.
Insert the cork.
Insert the bike pump needle in the cork, then pump the bike pump four or five times. Sometimes when you do this the cork will blow and you’ll have your reaction. Other times you’ll have to pump a few times more, then stop and pull the needle out.
WHAT TO EXPECT
Once the pressure is released—either because the cork blows or the needle is removed—a cloud should form quickly and dramatically in the bottle. Note: Aim the bottle away from you and anyone else.
No cloud? Your seal isn’t tight enough. Try a different cork, or use duct tape to tighten the seal.
BONUS: REVERSE THE EXPERIMENT!
Before the cloud disappears, put the needle back in and pump a couple more times. The cloud should disappear as quickly as it came. Release the pressure, and the cloud will form again.