In creating the new form 'fòk' the child has resorted to a process called back formation. There are many examples of this in the history of languages. For example, in English the '–s' plural was not as widely used as it is now. There was once a larger class of nouns like 'ox' which formed plurals like 'oxen' by adding '–en'. […] However, the '–s' ending was becoming so pervasively the sign of the plural in English by analogical extension from other plural forms that anything that didn’t end in '–s' was taken to be the singular.
- Romaine (1988), a pag.136