Note: If your water bottle happens to be full of water, drink up to empty it. It’s fine—even preferable—if there are still a few droplets of water left inside.
Put on your goggles.
Screw the cap on the bottle, but not all the way: The cap should be tight enough to make an airtight seal, but loose enough so you can unscrew it with a flick of your thumb. (You’ll figure this out after one or two tries.)
Grab the bottle at both ends and twist your hands in opposite directions. Twist hard—hard enough to create a narrow “waist” around the middle of the bottle. (Click to enlarge photos below.) Do you notice the bottle feeling warmer as you do this?
Point the top of the bottle away from yourself or anyone nearby. While holding tension on the twisted bottle, remove the cap with a sideways flick of your thumb. Caution: The cap will fly off rapidly. Never aim it at another person.
Watch carefully: You may see a small cloud of mist form near the mouth of the bottle after you release the cap. Note: If you can’t get mist to emerge from the mouth of the bottle, try adding a few drops of isopropyl alcohol, and then shake the bottle. Isopropyl alcohol’s lower vapor pressure will condense more easily than water vapor when you unscrew the cap.
If you happen to have an infrared thermometer, repeat the experiment and have a partner measure the temperature of the bottle before you squeeze it, as it is being squeezed, and then after the cap has been released. (Click photo to enlarge.)
If you want to do this Snack again, just blow into the bottle to re-inflate it and you'll be ready to go. The bottle can be used multiple times. Note: Do not re-inflate by mouth if you've previously used isopropyl alcohol in the bottle.