Halliday & Matthiessen (1999: 269-70):
In English, then, the metaphoric movement is from the logical towards the experiential and, within the experiential, from processes to things … a shift towards a more highly taxonomised way of meaning. But the basis for such a shift is found in the ordinary spoken language of everyday discourse. Consider the following series of examples:
You made three mistakes.That was his biggest mistake.Give it another big push.She gave him one of her most heart-warming smiles.Can't I have just two little bites of your cake?That last dive was the best dive I've ever done.
All these are instances of grammatical metaphor, with 'mistake' (verb err), push, smile, bite, dive turned into things (nouns) and the 'process' taking the form of a lexically very general verb give, have, do, take, make which retains the full semantic potential of a figure (tense, modality, &c.). The effect of nominalising these processes is to open them up to all the 'quality' potential that is associated with things: they can be classified, qualified, quantified, identified and described. This range of grammatical metaphors has become fully codified in English and is, in fact, used by children almost from the start.