Use clear plastic cups and pour samples of various sugary drinks into each one, along with a tooth. Pour water into a final cup, and add a tooth. Number the cups and create a chart. Examine every tooth every day, making notations when each tooth begins to show black marks, the classic sign of decay. Track progress as the teeth continue to decay and keep detailed notes to determine which drink is most harmful to teeth.
Carefully weigh three teeth, one at a time, and make precise notes of the beginning weight. Place one tooth into a cup of sugary drink, one into a cup of sugar-free drink and one into a cup of water. Each day, remove all three teeth, dry them and weigh each carefully. Mark the weight of each every day and observe how much weight each tooth loses, on a percentage basis, each day.
This experiment uses eggs as substitutes for teeth. Place one egg into each of several measuring cups, then cover each completely with a different brand and formulation of toothpaste. Include a fluoride paste, an anti-plaque paste, a whitening paste and so on. Make sure that there is enough toothpaste to keep the egg from touching the bottom of the cup. Cover each measuring cup with plastic wrap and allow them to sit undisturbed at room temperature for four days. Remove the eggs and rinse off the the toothpaste, rinse the measuring cups and allow to dry overnight. Make sure to note which egg used each toothpaste. The next morning, fill the measuring cups with enough vinegar to cover the eggs and gently place them into the vinegar with a spoon, then place a spoon on top of each egg to keep it submerged. Fill another measuring cup with vinegar as with the other cups, then add a fresh egg that has not been protected as you did with the other eggs. Cover all measuring cups with plastic wrap and allow to sit overnight. The next day, remove the eggs and examine them. The unprotected egg should be very soft from the acid in the vinegar attacking the calcium in the egg shell. The others should show varying degrees of softening, indicating each formulation's ability to protect your teeth.
Do girls or boys get more cavities? Use your class's annual dental checkups as a contest. Make one chart for boys and one for girls, with name, date and number of cavities discovered at the checkups. As each child has her checkup, enter the date and number of cavities. Once everyone has had a checkup, add up the number of cavities and divide by the number of kids on each chart to find out if, on average, boys or girls have more tooth decay.