From Academic Kids
A jug is a container for liquid, with a handle and an opening for pouring or drinking from.
Most jug players produce sound by buzzing their lips near the opening of the jug. The pitch is controlled by the tension of the player's lips and the flow of air through them. The jug forms a resonant cavity to modify and enrich the sound from the pure "buzz". In this way a single jug can produce many notes. Some players augment this sound with vocalizations.
In addition to the most common ceramic jug, many different containers and materials have been used for musical jugs (glass jugs and bottles, plastic bleach bottles, tin kerosene cans, etc.). The jug is primarily an acoustic instrument, although amplified and "electric jugs" appear from time to time (such as in the 1960s psychadelic band 13th Floor Elevators).
Jugs will also produce sound at their main resonant frequency when air is blown across the top opening. This method is not commonly used for jugs, since it is relatively quiet and produces only a single pitch. (It is more typically used for playing glass bottles.) A larger internal space (volume of air) in the jug produces a lower musical pitch whereas jugs with smaller interior spaces produce higher pitches. Thus the single pitch of a jug played in this way may be adjusted by adding or removing a quantity of fluid (such as water or moonshine) from the inside the jug. Loudness is a function of the speed of the air blown across the top.
How to Prepare and Play a Jug (http://www.jugstore.com/jugs.html)