Billy has had the same mobile hanging above his crib since he was born. when his mother lays him down for a nap, billy does not look at the mobile likely because he has become to what was once a novel stimulus

It is likely that Billy became habituated to the mobile. 

Habituation in this context refers to the phenomenon wherein after repeated exposure to a certain stimulus (toy or object), infants no longer show interest and pay attention to the stimulus. Instead, infants are captivated and stare longer at new or unfamiliar objects. In this example, Billy no longer stares at the mobile since he is familiar with it.