Plastic Shepherds Mouth Whistle used for years by professional dog trainers.

Shepherds Whistle Instructions.
1. Place whistle in mouth with the curved open edge facing outwards resting on your tongue.
2. Hold in the mouth in such a way as to create a gap underneath the whistle. The back of the whistle will sit on the back of the tongue.
3. Blow and by varying the blow, you will create different sounds.
4. To create an even wider range of sounds, move the tongue up and down reducing and widening the gap underneath the whistle. N.B. if you place your tongue against the lower hole you will silence the whistle. It doesn?t matter which way up the whistle is placed.