Acrn
Acrn is a whistle that has to be held in a particular way with both hands in order to create the right shaped cavity to allow for an airflow that makes a whistling sound. If used properly this whistle can create very high volume sounds. As a keychain it could be a good outdoor/hiking safety whistle that could last for hundreds of years if made out of metal. It is very hard to break, and it's open design cant get clogged, which could affect the functionality of typical whistles. It was inspired by a trick my parents taught me as a kid, which was a way you could use acorn caps as a whistle. I wondered why nobody had turned it into a product. I currently have several functional prototypes.