Ever "twang" a tight clothesline, a rubber band, or some other taut string, strap, or wire? It made a funny noise, and, if you touched it again, you felt it vibrating. That vibration sent through the air, created the sound.
If your string-thing was in contact with something else--maybe a post or a box--it made that vibrate, too, creating a louder and fuller tone.
Congratulations! You've just discovered a guitar.
A perfect mug for those that have a liking for musical instruments, most specifically, guitars.
