Tankard of sobriety was a magical tankard.
The tankard was a regular tankard similar to those found in any tavern, but it had one key distinguishing feature. On one side of the tankard, there was a face with a stern expression carved into it. The tankard prevented the drinker from becoming intoxicated. Any ale, spirit or wine poured into it had no deleterious effect. The tankard did not protect the drinker from poisons or toxins, nor did it have an effect on magical liquids such as potions.