>>By the user. Educate the user by telling that an item is selected by a single click.
>
>I have found that relying on educating the user is (1) a frustrating and usually fruitless exercise, and (2) a poor substitute for better software operating design.

I'll add a bit of humor here.

Ever see those web pages that say "do not click a second time, or you'll be billed twice" ?

Try implementing that - THAT will teach the users! :)

(just kidding)