Natural Frequency Introduction

All objects that oscillate have something called a natural frequency.

At this frequency they will experience resonance and start oscillating with increasing magnitude towards infinity, or in the case of something like a bridge until it breaks.

In another instance, a snare drum can be sitting motionless, but if a saxophone player near it plays a certain tone that happens to match its natural frequency (or a multiple of it) then the snare drum will start vibrating.