Final answer:
When pursing your lips to whistle, you engage the buccinator muscle. This muscle compresses the cheeks and is critical for facial expressions and oral functions such as whistling and chewing.
Step-by-step explanation:
If you purse your lips and whistle, you are using the buccinator muscle. This muscle, which composes a large portion of the face, is responsible for compressing the cheek and allows for actions such as whistling, blowing, and sucking. The buccinator muscle is also involved in the action of chewing and contributes to keeping food within the mouth when eating.
Other muscles in the area include the orbicularis oris, which surrounds the mouth and controls what comes in and goes out, and the masseter muscle, which is involved in the action of chewing. However, for the act of whistling, it is specifically the buccinator muscle that is engaged.