Dad usually falls asleep right after Thanksgiving dinner and Mom and Aunt Velma start to fight over the stress of cleaning up after dinner. Dad doesn’t like to be told what to do, but he needs to help clean up. If Dad helps to clean up, then Mom and Aunt Velma won’t upset Grandma due to their excessive bickering.
When Dad helps to clean up after dinner, Mom and Aunt Velma stop bickering (Negative Reinforcement). After cleaning up, Dad can sleep (primary reinforcer). Sinceo Dad now helps to clean up every Thanksgiving, it is fixed-interval.