A glass can vibrate, and like many objects it has natural frequencies at which it vibrates especially easily. A sound wave at one of those frequencies can repeatedly transfer energy into the vibration. If the driving frequency matches a natural frequency, resonance can produce a much larger amplitude. A powerful, steady sound may then make the glass vibrate strongly. Breaking a glass requires the stress from those vibrations to exceed the strength of the material, so an ordinary speaking voice is very unlikely to do it. The demonstration is dramatic, but the underlying idea of resonance also matters in musical instruments, buildings and engineering.

Engineers use this relationship when choosing shapes and contact areas. Ships, hydraulic systems, tyres, foundations and cutting tools may look unrelated, but each is designed around how forces are spread through a material or fluid. Similar principles appear in nature too, from broad animal feet on soft ground to floating ice changing the conditions at the surface of a lake.

Waves transfer energy and information without transferring matter from source to receiver overall. Frequency tells us how many oscillations pass each second, wavelength is the spacing between matching points, and wave speed links them through v = fλ. Different frequencies interact with materials differently, which is why engineers can choose particular parts of the electromagnetic spectrum—or particular sound frequencies—for sensing, communication, heating or imaging.