Obtain some powdered milk. Prepare a cup of powdered milk in a jar, by adding water to it according to directions.

Place an equal amount of the powder into a second jar, but keep it dry. Cover the jars and place them on a windowsill.

Open the jars after two days to see if they smell the same. So what if they don't?

The liquid milk may have a serious horrible stench to it. Why is that? Some scientists have estimated that one drop of milk might contain as many as 100 million bacteria. (They say it "might," because who wants to waste their time counting to 100 million? It sounds very impressive, and based on the smell, it is probably true.)

You will realize that the lack of water had an effect on the growth of bacteria in the milk. It probably should also make you realize that liquid milk should be kept very cold, and should be discarded after the "expiration" date on the container.

Do bacteria need to eat? Yes! What do they eat? I guess that depends upon what kind of bacteria they are. There is some horrible bacteria that seems to eat flesh. That is pretty nasty. It is apparently a mutation of the same bacteria that gives us sore throats and other minor stuff. It is very important to be careful about food, cleanliness,
and our immediate environments.