Resonance
Resonance is what happens when a sound or vibration makes another object shake on its own.
A wine glass sits on a table. You run a wet finger around the rim at just the right speed. The glass starts to hum and ring louder and louder. You are pushing it at its own natural rhythm, so the vibrations build up instead of dying out.
Explaining resonance by grade level
Rub your wet finger around the rim of a glass. The glass starts to hum. That hum happens because your finger pushes the glass at just the right speed. The glass shakes back and forth and makes a sound you can hear.
Projects that explore resonance
Every goblet has a natural frequency — its own built-in pitch. Tap one and record the sound with a microphone and oscilloscope to measure that exact frequency. Then place the goblet near a loudspeaker and slowly tune the tone to match. When an outside sound gets close enough to the goblet's natural frequency, the vibrations grow stronger and stronger. In one test, a goblet vibrated at about 800 cycles per second. When the speaker matched within 0.5 Hz, the glass shattered — showing how precisely this effect depends on frequency alignment.
Run a wet finger along the rim of a glass and the glass vibrates, producing a clear tone. That vibration is the glass shaking on its own in response to the motion of your finger. Change the water level and the pitch shifts, because the water changes how the glass vibrates. With less water the note goes one way; with more it goes the other.