Knowing relationships is important in learning metric. It's difficult in non-metric units.
One of the best ways to learn to think metric is to learn that a millimetre of rain in one square metre equals one litre of water which weighs one kilogram This means that one cubic metre of water equals one metric tonne. How nice it would be to have river flow, petroleum, and other liquids reported in these terms. People would learn the relationships very quickly and get a feel for density of fluids and to compare them. It's fundamental. Regards, Stan Doore
