A small round cantine, that is strapped around its wearer with a leather strip. It has the ability to never run dry. It will be able to contain any type of liquid such as acid or lava. It will also never run out.
Magic should have side effects, both expected and unexpected - fortuitous and deplorable. Expected side effects add anticipation, while unexpected ones increase the drama of the scene. The result can be comedic or dire, it dpends on which circumstances work best for your game.