Tuning forks are fascinating, simple tools that don’t get quite enough appreciation.
Striking a prong against a hard surface causes the fork to hum in a specific pure frequency. This frequency is calibrated based on the fork material and size.
The hum is produced by the vibration of the prongs which create tiny, alternating high and low pressure regions of air which radiate out. This is what we call sound.
Why do tuning forks have two prongs? If it had just one prong, essentially a metal rod, the force of striking one end would cause an opposite force on the other end, where your hand would dampen the vibration quickly.
But with two prongs, the force travels through the bar and is exerted in the second prong. The handle becomes the neutral point so the sound lasts much longer.
Tip: Hold the handle against a hard surface for maximum ring time.