Ben wants to know if the amount of air in a basketball affects how high the basketball bounces. He fills three basketballs with air and drops each one three times. Which part of the experiment should Ben keep the same for each ball?

The questions have been asked in several important state and regional science exams in the United States. For example: TAKS-Texas Assessment of Knowledge and Skills, New York State Educational Department Regents exams, California Standards Test, etc.