Sometimes an experiment can produce the results that a scientist wants it to produce.

If you are the batter, and you believe that a warm ball will travel farther than a cold ball, you may swing harder when you know that a warm ball is being pitched.

If you are the pitcher, and you believe that a cold ball will travel farther, you may pitch the cold ball so that it is easier to hit. How can we be sure that neither the pitcher nor the batter is subconsciously changing the results of this experiment? The answer is called a double-blind experiment. In a double blind experiment, neither the experimenter nor the test subject knows which condition is being tested. Keeping the batter from knowing the temperature of the ball should be easy. Keeping the pitcher from telling a warm ball from a cold ball is almost impossible.

Hitting a Baseball off a Batting Tee

How could you use a batting tee to make this a double-blind experiment?