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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. 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. 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. 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. 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. 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.
Obtain an empty plastic bottle. Remove the label from the bottle. Squeeze the bottle in the middle and twist. Twist the bottle about 4 to 6 times. Untwist the cap with your thumb. Watch the water vapor.