A double bubble sheet is used to compare and contrast two ideas, people, events, or places.
It is important that the differences and similarities go beyond the surface and show depth of thought. As an example, if you were to compare Rosa Parks and Dr. Martin Luther King Jr, I would not want to see a difference stating one is a woman and the other is man. A better example would be that Rosa Parks was a symbol for the civil rights movement, where as Dr. Martin Luther King was the leader of the movement.
The spot where you put your differences should also be matched up in location. Using the example above, if you put Rosa's difference on the top left, then Dr. King's matching difference should go on the top right. Or if you put Rosa's difference in the middle left, then Dr. King's matching difference goes in the middle.
Finally, no "not" statements. I don't want to see differences such as "Dr. King was born in Atlanta, Georgia." and "Rosa Parks was not born in Atlanta, Georgia."