h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Clay soil is either very, very wet or very, very dry. The transition state between exceedingly wet and exceedingly dry (or vice versa) is hypothetical, and has not been observed in objective reality.

Clay soil is also omnipresent whenever one is attempting to plant a shrub, tree, or flower to whcih clay soil is anathema. We believe this is because Garden Centres have signed a pact with the devil to enable them to sell soil additives and improvers.

Attempting to walk in clay soil is not advised. You would have more success trying to yomp across an entire landscape covered in warm treacle pudding. Clay soil's affinity for footwear was probably an inspiration to Imelda Marcos. The winner in the shoe race is, however, guaranteed to be clay soil, since it has less incentive to stop that the energizer bunny.

Clay is also used for modelling... but do not be tempted to make anything important with the wodge of aggregated silt that you just dug out of the vegetable garden. Clay suitable for fine china, or even for kindergarden pottery, is a much rarer beast, and is found only in inaccessible places, protected by acrea after acre of the more common, shoe-grabbing variety.

Clay is brown, or red, or yellow, or even slightly green. Clay is also ochre which is just an old fashioned tautological way of saying it's earth-coloured. It IS earth (or at least most of the earth I've ever seen).