Abby's new puppy cries each night when left in her crate. when abby picks up the puppy and holds her to her chest the puppy stops crying and falls asleep. however, after she puts the puppy back in the crate the crying begins again. after a few nights, abby suddenly realizes that her heartbeat is the dog fall asleep. she places a ticking clock next to the crate and the puppy goes right to sleep at night. abby's realization is an example of: