Why do my cats sometimes come face to face and sniff each others noses?

Friday, July 30th, 2010 at 3:10 am

There both male and usually tolerate each other, but 1 cat sometimes tries to attack the other like a bully. But as cats being cats, they like there own space so perhaps this nose to nose encounter is instinct?
why would they greet each other? they usually despise each other!