Zeigarnik Effect

"An ignored task is just an ignored task. A task in which the first step is done is an uncompleted task, and people can't let an uncompleted task go. This has its own name: the Zeigarnik Effect, named after Bluma Zeigarnik, a psychologist who studied the fact that waiters had better recollection of unpaid orders and students had better recollection of uncompleted work. An uncompleted task sticks in the mind. By starting the task, you make it so your target can't just forget that they have something to do."