Curl your tongue back and use the first two fingers of each hand to press it down firmly. Then, place your index fingers on the sides of your mouth to block the airflow and fold your lips over your teeth. Take a deep breath before pressing down on your first knuckle, and exhale forcefully. This drives air over your tongue and through the small gap between your fingers. By adjusting your fingers, lips, and tongue, you’ll eventually discover the right setup to produce a loud whistle.