Because if you are, you're picking up a dog.
From first episode of season 8 of friends. If you're into geography/history you probably get it. (Check/Czech/Yugoslavia)
My friend dared me to drink a sort of dangerous chemical in Science class and I thought she was talking about the olive oil or vegetable oil or whatever that was near it, so I said this, and we couldn't stop laughing about it and kept referencing it all class. See also Chemicals, Italian, Science.