You can turn nearly any object into a speaker by using a piezo disc and a handful of additional components. While this may seem like magic, there is actually a rather simple technical explanation. By driving a piezo disc using an amplifier, the disc vibrates and then resonates the sound wave through whatever object the disc is attached to. When the object vibrates, it disturbs the air and makes sound. This is not only a fun trick, but also allows for a lot of interesting experimentation and creative projects.
Turn Nearly Anything Into a Speaker: 13 Steps (with Pictures)