Did you know that millions of tiny creatures called bacteria help make that creamy, tangy lunchtime treat you know as yogurt? Now you can observe and taste all their hard work for yourself by following this experiment. (Talk about a culture shock!)

• 1 container of commercially-prepared plain yogurt
• 1 quart of skim milk
• a measuring cup
• a candy thermometer
• a saucepan
• a clean quart-sized glass container
• a towel

1. Warm the milk in a pan until a skin forms at the surface (the thermometer should read 160 degrees F. at this point). This kills any germs that may spoil the milk before the yogurt forms. Then cool down the milk to 110 degrees F.
   
   
2. Take 1/2 of the container of yogurt and add an equal amount of warm milk to it. Mix this thoroughly, then add the rest of the yogurt and stir. You've just created your "starter" culture.
   
   
3. Gently blend the starter culture with the rest of the milk in the pan.
   
   
4. Pour the entire mixture into the glass container and wrap a towel around it to seal in the heat. Let the yogurt culture stand undisturbed at room temperature, covered or uncovered, for 8-12 hours.
   
   

  You have a jar full of homemade yogurt! You can tell when you're finished by tilting the container gently; if the yogurt slides away from the side of the jar in one piece you're finished.

  Put the finished yogurt in your refrigerator to stop the bacterial growth. Then eat the yogurt plain or mix it with honey or fruit preserves. It won't be as thick as the commercially-made variety, but it's a supply of yogurt you can truly call your own!

  Your yogurt is alive with the workings of tiny one-celled organisms known as bacteria. When you added yogurt to your milk, you were adding a type of bacteria called acidophilus to it. These bacteria feed on lactose, a sugar found only in milk. They also produce a waste product called lactic acid as they feed.

  By letting your yogurt mixture stand at room temperature for several hours, you created an ideal environment for the growth of even more lactic acid bacteria. As these microbes multiply, they produce lots of lactic acid. This acid givesyogurt its distinctively sour taste and thick, custard-like texture.