In this, you need two cups of water and four or more people. The people in the skit all line up in a row in front of the audience. The first person in line has water in his cup and the last person in line has an empty cup with some water hidden in his mouth.
The skit starts off with the narrator saying something about this as a demonstration of how a person can brush his teeth when there is a shortage of water. The first person takes a drink of water from his cup and proceeds to brush his teeth with his finger. After a few seconds he stops and places his head against the ear of the next person in line and that person's cheeks begin to bulge out as the first persons deflate. This person (the second person brushes his teeth with his finger for a few seconds and then puts his mouth against the next person's ear with the same results and so on down the line until the last person in line is reached. This person upon finishing brushing his teeth releases the water he has in his mouth into the cup in his hand.