how to mark all checkboxes...

I have a list of checkbox...
and I want the first check do "check all / none"

[ ] click to check ALL answers
[x] blabla
[x] blabla
[x] blabla
[x] blabla

How I can do that?

thanks.

Javascript.

http://www.google.com/search?q=check+all+checkboxes+javascript