In the "American" system, the same word OZ can refer to weights and to volumes; in the metric system the basic word for weight units is gram and for volume units is liter--so in the American system, confusion is easy--throw a TSA agent into this, and we are in trouble.:D
[Side note: after the French revolution, there were plans to decimalize time; instead of 60 seconds per minute, one minute would have 100 units...]