You lunge at the man behind the counter, snapping your doghand jaws at his face and arms. You feel its teeth rip into his flesh and taste his blood, and you start to get a little carried away.
Before too long, you're leaning over the counter, smeared with blood, with nothing left of the cashier but a bloody puddle and a pair of sneakers. You let out a large blech.
Something's strange though, your doghand has grown, and now it's a whole dog's head at the end of your forearm, not just the jaw! It's gaining strength!