What you'll need:  

• 1 bottle of fluoride rinse
• 2 eggs
• 1 bottle of white vinegar
• 3 containers 

What to do:

Pour four inches of fluoride rinse solution into one of the containers and then place an egg in the solution. Let it sit for five minutes. Remove the egg. Pour four inches of vinegar into each of the remaining two containers. Put the egg that has been treated with the fluoride into one container of vinegar and the untreated egg in the other container of vinegar. 

What will happen:

One egg will start to bubble as the vinegar (an acid) starts to attack the minerals in the egg shell. Which egg do you think will start to bubble?

The fluoride becomes incorporated into the mineral shell of the egg, thereby protecting it from the acid. Fluoride from toothpaste, mouthwashes and professional treatments protect you teeth's enamel (outer shell) in the same manner. Your teeth are constantly under attack from acids in foods, drinks and bacteria naturally occuring in the mouth (plaque).