[ZenGum]

My favourite bit about metric is the cross-relations, as in:

1 cubic centimetre = 1 millilitre
1,000 cubic centimtres (10 x 10 x 10cm) = 1 litre
1 litre of water = 1 kilogram

so, one cubic metre = 1,000 litres = 1,000 kilograms (if its water).

And the metre was orginally defined as one ten millionth of the distance from the equator to the north pole along the Paris meridian, so the Earth is appx 40,000 km in circumference.

This makes all sorts of conversions and estimations easier.