Before we figured out the whole “electricity” thing, all instruments were acoustic in the sense that they could only be amplified by mechanical means. A pipe organ, for example, is able to produce loud notes with pressurized air. That technique can be traced all the way back to ancient Greece, when hydraulic actuation was first used to displace water in order to pressurize the air. Now, thanks to electricity, we can amplify any instrument quite easily. But there are still multiple ways to go about that. One method is with a piezo pickup, which you can use to turn just about anything into an amplified instrument.
In addition to piezo pickups, magnetic pickups and microphone-based setups are also very common. Microphones work in a pretty obvious fashion, and are usually used to amplify acoustic instruments like drum sets. Magnetic pickups are most often used on electric guitars that have metallic strings. As those strings vibrate, they induce a corresponding electric frequency in the magnetic pickup. That signal can then be fed into some kind of amplification setup, like a tube or solid state amp. Unlike those methods, piezo pickups work by picking up the vibrations moving through a material, which means they’ll work with just about any material that is hard enough to transmit those vibrations.
You’ll find piezo pickups on acoustic-electric guitars, violins, and many others. The piezo pickup, which can also be called a piezo sensor, essentially works like the inverse of a speaker. Physical vibrations, usually transmitted through direct contact, create an electrical signal that can be detected. For that reason, they’re often referred to as “contact microphones.” As is explained in this video, you can find piezo pickups for very little money. The output signal is very quiet, so you’ll need to feed it into some kind of amplifier. Afterwards, you can place your piezo sensor on just about any object and turn it into an instrument. The video demonstrates that fact on many different items, but the whole point here is to get creative and try it on things around your home!