Heel, click. Sit, click. Good dog.

Roanoke, VA - "Lilly, a 10-month-old yellow Labrador retriever, is standing at attention, her body alert. In front of her is her trainer, and Lilly knows she has a treat. The two remain motionless, eyes locked, until Lilly suddenly sits down. Just as her rump touches the floor, there's a "click" and the trainer gives Lilly the treat. It's called clicker training. The clicker is a piece of plastic, about an inch long, with a button that you can depress to make a clicking sound..."
Source: Roanoke.com, 9/22/2006