In Rules by Cynthia Lord, twelve-year-old Catherine often feels like she is being pulled between two worlds. At home, her younger brother, David, needs constant care and extra attention because he is autistic. He can also be embarrassing to Catherine because his behavior is sometimes loud, unruly, and just plain odd. There are times when Catherine would just like to hang out with her friends and go to school and pretend that she is just a normal girl in a normal family.

The events in the chapter "No toys in the fish tank" provoke this feeling in Catherine. Her new friend, Kristi, comes over to Catherine's house to hang out, but Catherine does not want to stay there, so she takes Kristi swimming at the pond instead. What's more, Catherine definitely doesn't want her mom to drive the girls to the pond. She doesn't want to have to deal with David. She would rather just hang out with a friend. Even though Kristi is only at Catherine's house for a couple minutes, she still notices that there is a plastic duck in the family's fish tank (hence the chapter title).

Even though Catherine doesn't tell Kristi about her feelings of being pulled in two directions, Kristi would understand, because she, too, feels like she is being pulled between two worlds. Kristi's parents are separated, and while she lives with her mother, she also spends time with her father. Her things are split between the two places, and she never quite knows where anything is or what she will need.

Indeed, both girls feel like they are "split down the middle" and that they don't belong completely to either of the worlds pulling at them.