Some bears kept in the zoo allow veterinarians to routinely give them total body checkups. these bears open their mouths for teeth cleaning and present their paws for nail clipping. your friend wonders how anyone could ever get these dangerous animals to be so cooperative without anesthesia. you, however, a psychology student, quickly surmise that the bears had most likely undergone

Operant Conditioning

Some bears must have had undergone operant conditioning that is why they open their mouths for teeth cleaning and present their paws for nail clipping.

Operant conditioning is a form of learning in which a reward or punishment is given for some voluntary behavior. 

Bears that have undergone operant conditioning are called food-conditioned bears. They are dangerous but learned to approach humans, or frequent human-occupied sites, in search of food (reward).

geologic time scale (for gradpoint) =)