"Let's both become the same animal," you say.
Your girlfriend agrees, and instantly a thick mist fills the room. When it clears, you've definately been transformed.... but where's your girlfriend? You call out to her.
"Ow!" she says. "No need to shout right in my ear!"
You look to your left, and see a bestial face beside your own, with a neck that's attached to the same torso as your own. But what are you, even? You hadn't actually thought of an animal yet!
"Hey! We're two-headed!" Your girlfriend exclaims. "Cool! And just like I wanted, we're a..."