It's good to practice good hygiene.
Bill has only been brushing his teeth for a short moment when his arm begins to get tired.
Bill wants to relieve his arm pain.
Bill switches hands and brushes his teeth with the other hand as his painful one rests.
Bill stops brushing his teeth and puts his arms down, goes to rest and forgets about brushing.
Bill kills all the bacteria in his mouth and his teeth remain healthy.
Bacteria build in Bill's mouth overnight beginning the process of forming a cavity.