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You can use either a metal or a plastic bottle cap, but since the cap will be going into your mouth, you need to make sure that it has been thoroughly cleaned, first.  Metal bottle caps are shallow and tend to produce more subtle, natural looking fake dimples. The ridges can be painful, though, especially if you have sensitive cheeks. Plastic bottle caps come in a few different sizes. If you want really exaggerated dimples, use a soda bottle cap, since these tend to be the deepest types. For a more subtle option, though, grab a shallow water bottle cap.  The solution should contain about 1 tsp (5 ml) salt for every 8 oz (250 ml) of water. Soak the cap for 15 minutes before rinsing it, drying it off, and placing it in your mouth. You could clean the bottle cap with soap and water, but make sure that all the soap has been completely cleaned off before placing the cap in your mouth. If using this technique, you should only create one dimple at a time. Bottle caps produce dramatic fake dimples, and having two dimples like this at once can overwhelm your face. Open your mouth wide and place the bottle cap in inside, wedging it in between your your cheek and your teeth. Make sure that the opening of the bottle cap faces your cheek and not your teeth. It may take a little work before you figure out the right placement, so play around with it a little. If you have a hard time figuring out where to place the cap, take it out of your mouth and smile in front of a mirror. Look for the outer creases the form on your cheeks when you smile. The cap should be placed just to the outside upper corner of the crease on one side of your face. Press on your cheek from the outside while simultaneously sucking on your cheek inward from the inside. Your cheek should firmly indent itself into the opening of the bottle cap.  If done right, you should even hear a small suction noise. Be careful as you do this so that you do not accidentally inhale and choke on the bottle cap, especially if you are using a small metal cap. This fake dimple technique does not really work when you are in front of someone in person, but if you want to create a fake dimple for a photograph, it works well. Take a few pictures from a few different angles until you find the look you like best. A popular way to pose with this fake dimple is to cover your mouth from below and turn your face so that the “dimpled” cheek faces the camera. Smile slightly, but do not allow the pressure to loosen from the bottle cap inside your mouth. This pose really accentuates the fake dimple while concealing any lines or bulges from the bottle cap, and it can also hide the fact that your mouth is not smiling quite as widely as it would usually need to if it were producing dimples that deep naturally.
Wash the bottle cap. Position the bottle cap in your cheek. Squeeze your cheek into the opening. Strike a pose.