Here are the answers

Why the English ride on the left? Why do people say toasts on important occasions? Why do we say good-bye?
 
Find all the answers in a book called Now you know, the book of answers. This is one of the best books I've ever read. It's not a big book and is fairly simple-written.
 
One of the answers from the book: Why do we say 'so long'? answer : British soldiers who went to Arabic countries heard local people saying 'salaam'. Trying to immitate them, they changed it to 'so long' and now it's been used by all.