my problem with the american system is you never know what the fuck they are talking about. for example, is a cup 236.6 ml or 240ml or 250ml?  is an ounce a measure of weight or volume? is it a troy ounce, avoirdupois ounce, or imperial ounce? is a ton a long ton, a short ton, or a metric ton? no one know the answer to these question. the terminology is ambigous because there is too much historical fuckery to parse through. thank god for the french and their metric system.
United States