Impromptu kazoo at the chocolate factory

We made kazoos from blades of grass to entertain ourselves when the coach needed fixing outside the chocolate factory in Deauville. You pick a wide blade of grass and wedge it between your thumbs, securing it at both ends. You blow over the grass blade. The air flow is split by the grass blade. As it passes the far edge of the grass, tiny whirlpools of air called vortices form. This is called vortex shedding. They push the grass blade first one way and then the other way making it vibrate. If the frequency of vibration matches that preferred by the length of grass, then resonance occurs and the vibration is enough to be heard.