A student noticed that when a dog is cut, the dog periodically licks its wounds. Usually after a few days, the wound begins to heal without ever showing
signs of infection. The
following steps outline the student's line of reasoning:
1. I wonder why the dog's wound doesn't become infected.
2. The dog's saliva must prevent the growth of infection-causing bacteria.
3. I'll obtain a bacterial culture and grow the same kind of bacteria in two identical
culture dishes. Once the bacteria start growing, I'll add dog saliva to only one of the
dishes and leave the other alone. I'll cover both dishes. Then I'll observe what
happens each day for a week.
4. Even after adding the dog saliva to one of the dishes, the bacteria continued to grow
in both dishes over the course of the week. However, the bacteria in the treated dish
grew more slowly than the bacteria in the untreated dish.
5. I think I'll try something else. I'll start with two identical culture dishes, as before, and
use the same kind of bacteria in each dish, but this time I'll treat one dish with dog
saliva before I add the bacteria. I'll observe what happens each day for a week. 12.
For the dog scenario above, what is the control
Look?