Butts.

If Tina Belcher were a (strange looking) cat, she would go around happily sniffing other cat butts. Which is totally normal for a cat.

Before I owned more than one cat, I thought butt-sniffing was just a dog thing. It is not. My cat, Ritsuka, is especially interested in the smell of other cat butts, and will sniff with such relish and intensity that my other cats will sometimes get offended and smack him right in the face. All four of them take part in butt-sniffing, but Ritsuka can’t seem to quite grasp the etiquette involved that the others have mastered.