Cats are much harder to train than dogs.

This formidable footage is over 6 minutes long. Be patient and watch all of it if you really want to see how Pavlov's "classical conditioning" works.

Short biography:

"Ivan Petrovich Pavlov (Russian: Иван Петрович Павлов) (September 14, 1849 – February 27, 1936) was a Russian physiologist, psychologist, and physician. He was awarded the Nobel Prize in Physiology or Medicine in 1904 for research pertaining to the digestive system. Pavlov is widely known for first describing the phenomenon now known as classical conditioning in his experiments with dogs."

Excerpt from http://en.wikipedia.org/wiki/Ivan_Pavlov