In the metic system one millilitre of water occupies one cubic centimetre, weights one gram, and requires one calorie of energy to heat it up by one degree centigrade – which is one percent of the difference between its freezing and boiling points. And an amount of hydrogen weighing the same amount, one gram, has exactly one mole of atoms in it. Simples!
Metric
And to think I spent all that time learning about rods, poles, perches & chains, none of which have stood me in any stead whatsoever – except from a fishing perspective (chains excluded)!