One of my favourite jokes.

A Scottish guy walks into his bedroom (where his wife is reading in bed) with a sheep on a leash and says:

"This is the cow I f--- when you aren't available".

His wife looks up & says:

"If you had any brains, you'd realize thats a sheep, not a cow".

He replies to his wife:

"If you had any brains, you'd realize I wasn't talking to you".