This is like a juice box except it’s thinner, so it can slide into a pocket easily, and the straw is built-in and never leaks. It appears to be made of some kind of cardboard, just like its mundane counterparts, but Convenience Juice is much more durable and resistant to damage. You’ll have a hell of a time cutting into the thing, for example, and even if you do, it won’t leak. Liquid only ever comes out of the straw, and only then if it’s going into a living human mouth.
Furthermore, it never runs out of juice. Further furthermore, the juice is whatever flavor you want. Apple, orange, grape, etc. Whatever you feel like, you can mentally request it before you put your mouth on the straw, and then that flavor of juice will be dispensed to you. It has to be fruit juice. It won’t dispense anything else.
The first Convenience Juices were limited, in that they would only dispense so much juice in a 24-hour period. These were (and still are) generally geared towards children, so they don’t make themselves sick drinking too much juice. Later, versions for adults were created that removed the limit.
Convenience Juice was invented in 1980 by Jane Gross, a kindergarten teacher and amateur artificer. It was a rare case of a magic item developed by an amateur that became popular worldwide, particularly in the education industry.