Why Dogs Sniff Each Other's Bums

(as told to me by an old Russian man at the off leash park this morning).

Me: "Good morning"
Old Man: something that I think was good morning
Old Man: says something else
Me: "Pardon?"
Old Man yells louder: "Has anyone ever told you why dogs sniff each other's behinds?"
Me: "Um, no..."
Old Man: "It goes back to Creation, God was talking to the dogs and one of them let a very loud one rip. God said "Who was that?", and none of the dogs answered. He asked again "Who did that?". When none of the dogs answered again he banished them from paradise saying "You can't come back until you find out which one of you did that"."
Me: nervous laugh "I've never heard that version before"

So my friends, dogs are sniffing each others' behinds not to say hello, but to figure out who farted in front of God.