Grammaticalization is the process of turning an ordinary word into one with a specifically grammatical function. Ordinary (or lexical) words include nouns like head, foot, dog, adjectives like bad, big, and verbs like come, go, eat. Grammatical (or functional) words are those like the, is, from, when.

Already in the above list we find some that have been partly or wholly co-opted into a grammatical function in English alone. Sometimes these still feel like metaphor or simple extension of meaning: the head of the river, the foot of the stairs. But in other cases they've been full grammaticalized: ahead; or going to to make a future tense. When you're going to sing, you're not going anywhere, not even metaphorically.

The loss of any sense of the original word in its true meaning, head or go, has been called semantic bleaching. It's one characteristic of grammaticalization. Another is phonetic weakening. Grammatical forms, being more commonly used than ordinary words, are under greater pressure and more likely to change. This can actually be contrastive: we usually say it with a reduced form in I'm gonna sing, but can't say that in I'm going to Paris.

Prepositions are intermediate between lexical and functional words. As a class they're open to new membership, occasionally, and do have a core of clear meaning. Often prepositions come from grammaticalized compounds: ahead of, beside, below, in front of, facing, American English in back of, and others where we might feel the process is not yet complete: in the face of, in the eye of.

Probably every language co-opts words like this. It seems very natural. I have just acquired a fascinating book, World Lexicon of Grammaticalization, 2002, ed. Bernd Heine and Tania Kuteva, CUP. I had no idea there was such a thing, and this appealed to the butterfly collector in me.

'Head' can become 'top' or 'on' (the head of the class, the head of the page); that's happened in Shona, Ancient Egyptian, Welsh, and a whole bunch of others. It can became 'forward' (headlight, ahead); they quote a Maasai example for that. One common use is to stand for the person and thus come to mean 'self', or reflexive: this has happened in Abkhaz, Basque, Fulfulde, Georgian, Hausa, and Mordvin. In fact, 'head' is one of the most common sources for the meanings 'self' and 'up'.

Adjectives meaning 'bad' coming to mean 'very' are attested among others in English and German: Es schmeckt furchtbar gut, It tastes awfully good. This feels like it's quirky, but it's found in Africa and New Guinea too.

The Chinese perfect tense marking le comes from a verb liao, 'finish'. This is common, as is the use of come and go: French je viens de manger 'I've just eaten', je vais manger 'I'm going to eat'.

Something that is already more functional than nouns or verbs can be pressed into service for another function, with more semantic bleaching and phonetic reduction. There means a place, but it is an empty word in expressions there is, there are. These can contrast: a place reading in There's the book you wanted and a phonetically weaker form in There's a book about it in the library.