Latisha’s new puppy cries each night when left in her crate. When Latisha 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, Latisha suddenly realizes that her heartbeat is helping the dog fall asleep. She places a ticking clock next to the crate, and the puppy goes right to sleep at night. Latisha's realization is an example of