Ruby, !! operator (a/k/a the double-bang)

Possible Duplicate:
What does !! mean in ruby?

Hi,

I'm new to Ruby and can't find anywhere description of what "!!" means.

Here's an example:

def signed_in?
  !!current_user
end

If this is a double negative, why not to say:

def signed_in?
  current_user
end

Please help.