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SEE HOW DIRTY WE ARE!

Prepare five or more sterilized petri dishes or test tubes containing a nutrient medium. Do not open them until you have decided what to do with each.

You might:

(1) let a fly walk across one;

(2) place a hair on one;

(3) touch one with the tip of a lead pencil, a finger, or a coin;

(4) cough or sneeze into one;

(5) place food, such as a drop of milk, a piece of meat, or a piece of cheese onto another;

(6) or expose several to the air for different lengths of time.

Soil samples can also be tested.

Cover, label, and seal each container with adhesive tape.

(Caution: Once sealed, do not open a container again. This will prevent exposure to pathogens that could develop and be dangerous. When the study is completed, carefully destroy and dispose of the colonies by boiling them in a large quantity of water or by soaking them in a strong disinfectant for twenty-four hours.)

For alternative experiments you might compare:

(1) a five-minute exposure of a dish in a quiet room without people to a five-minute exposure of another dish when people are moving about;

(2) a washed finger with an unwashed finger.

For each experiment, maintain an unopened container for comparative purposes.

Isn't there a comic strip in the newspapers with a kid named "pig-pen?" Everywhere he moves, you see specks falling off him, and people turn away when he comes around. When we start growing stuff that collects in a petri dish that was set in a room with people moving about, we will envision our own selves walking around with specks falling from our bodies! Yuck! Maybe we'll learn to be more careful about personal cleanliness and sneezing and other stuff.

 
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