My BanJello, I'm not bragging about how it makes horrible noise. I am bragging about how it works to make sound though.
First of all, when a string is plucked, struck or hit, the string vibrates and waves push into the hollow body, making the waves bounce off the sides of it, which creates louder waves. The waves vibrate out of the cavity, which ring around as sound.
To make the sound higher or lower, simply create the string shorter by putting a finger on a part of the string down the neck. The more down the neck, the higher the note. Then pluck it to create the higher sound.