How to Make a Water Bottle Cap Pop off with Air Pressure

Using the Twist Method

Obtain an empty plastic bottle. This experiment works best with a bottle that has relatively thin plastic. You need to be able to twist the bottle, so bottles made of thicker plastic will be difficult to use. You also want to use a standard size (16.9 oz or 500 mL) bottle or larger, not the miniature ones. You want the bottle to be mostly empty: if you leave a small amount of water in the bottom of the bottle, you will see a cloud of water vapor form at the end. Bottles with smaller lids also tend to work better.

Remove the label from the bottle. While this step isn’t entirely necessary, it can make it easier to twist the bottle for the experiment. Recycle the label when you’re done with the experiment.

Squeeze the bottle in the middle and twist. In order to build up the air pressure inside the bottle, you need to decrease the amount of space available by crushing the bottle. Start by squeezing the middle of the bottle and twist the bottom. Starting the twist can be difficult. In some cases it can be easier if the bottle starts out slightly deflated. Remove the cap, squeeze the bottle a bit and replace the cap while keeping the bottle slightly squished. Make sure the cap is tightly screwed on to the top of the bottle before beginning this step.

Twist the bottle about 4 to 6 times. As you twist the bottle, you will notice it will become more difficult. Twist the bottle as many times as you can until you can’t twist it anymore. If you are not strong enough to do it, ask an adult for assistance. Keep the bottle pointed away from your face and others while you are twisting the bottle. While it is unlikely for the bottle to pop during this step, it is possible.

Untwist the cap with your thumb. Grasp the bottle tightly against your stomach with the cap facing away from you. Use the side of your thumb to quickly twist the cap off. Make sure you fully twist the cap off, or the cap won't pop out. If done successfully, the cap should fly across the room. Another way to pop the cap off is to hold the bottle between your legs and then twist it with your hand. If the cap does not fly across the room, there wasn’t enough air pressure built up inside the bottle. Untwist the bottle, blow into it to refill with air, and try again.

Watch the water vapor. When the cap flies off you may notice that a cloud of white vapor is coming out of the bottle as well. This is water vapor that is formed when the cap gets released. As the pressure builds inside the bottle, the water molecules get squeezed together. When the cap goes off, the temperature inside the bottle cools causing the water molecules to condense into vapor. This is also how clouds form in the sky. At cooler temperatures, the water vapor forms around dust particles to condense.

Reusing the Bottle

Untwist the bottle. Once you have launched your cap, you can do it over and over again until you get bored. Untwist the bottle until it is back to its original shape. The bottle will be a little bit crunched but it will still work. If the plastic cracks at any point in the bottle, it will no longer work for this experiment.

Refill the bottle with air. Place your mouth around the top of the bottle and blow into it until the bottle puffs up again. You will hear a crackling sound as the plastic inflates. It does not need to be completely full, it just needs to have enough air inside it to compress. Don’t share bottles with multiple people to avoid the spread of germs.

Recap the bottle and twist. Replace the lid on the bottle and squeeze in the middle as you did before. Repeating the same steps, twist the bottle until you can’t twist it anymore. Remember, the more twists you can get, the more pressure that will build up inside the bottle. The bottle should begin to twist more easily the more times you use it. At some point, the plastic will likely crack and you will not be able to use that bottle anymore.

Launch the cap across the room. Position the bottle against your chest with the cap facing away from you and twist it off. Watch it shoot across the room again as the air pressure gets released. Repeat as many times as you’d like or until the bottle cracks. Remember, never aim the bottle towards a person or an animal. Avoid pointing the bottle at your face as well.

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