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Use a washcloth or paper towel. Find a twig. Make do with your finger.
A rough washcloth will do a better job of cleaning, but a paper towel will do if a washcloth is not available. Wrap the washcloth or paper towel around your index finger, dampen it, and add toothpaste if you have some. Brush your teeth as if using a toothbrush: begin at the gums and work down, cleaning each individual tooth with a circular motion. Don't forget to brush your tongue. Rinse your mouth thoroughly when done, swishing back and forth. Before there were toothbrushes, most people brushed their teeth with twigs. In many parts of the world, they still do, using twigs from oak, arak, or neem trees. Research shows that twigs from the arak tree contain natural fluoride and antimicrobial agents, and brushing with them is as or more effective as brushing with a toothbrush and toothpaste. Pick a young, flexible branch about 6 to 8 inches long (15 to 20 cm). You want one without real bark, only a thin skin. Peel the skin off and chew on one end of the stick until the fibers separate, turning the end into a little brush. Use it to brush your teeth. You can also use a toothpick to clean between teeth, but use caution so as not to hurt your gums and make them bleed. If no paper towels, washcloths, or twigs are at hand, you can always use your finger. Be sure to wash your hands very well first, then use your index finger as you would a toothbrush: begin at the gums and work down for the upper arch and work up for the lower arch, cleaning each individual tooth with a circular motion. Be sure to rinse your finger before moving from your top to bottom teeth, and from the front to the back of your teeth. Rinse your mouth thoroughly when done, swishing back and forth and from one cheek to another for at least 30 seconds.