Pavlov's dog salivated to the sound of a bell because

The dog was conditioned to it. The unconditioned stimulus (food) triggered the unconditioned response (dog salivating). By ringing the bell before bringing the food, the bell (once a neutral, but now a conditioned stimulus) triggered the dog's salivation. The dog's response is a conditioned response because he learned to associate the bell with food.

This is a time line from 685 to 978

Orbit is represented by the blank space