The Richness of English - one 'challenging' question.

Quite often at the beginning of the school year, I like to challenge my French students of English and ask them the following question:
Where does the difference between the following words come from? :

• cow and beef
• calf and veal
• swine (or pig) and pork
• sheep and mutton
• hen and poultry (although chicken is also used for the meat, ‘poultry’ is never used for the living animal)

Obviously they can figure out that one type of word is for the living animal when the other is for the meat you eat. (although chicken is also used for the meat, ‘poultry’ is never the living animal). But why is that? Where does it come from? Of course, as a teacher of English, I ought to know, (one of the golden rule of teaching is to only ask questions whose answer you know)... but do you?

NOTE: And please, don’t Google or Yahoo or engine-search it, that would be cheating. (I will post the answer in a couple of days.) I'll give you a hint - the answer has to do with a guy called 'Bill'.

NOTE2: I also like to ask them how many tenses there are in English conjugaison...