Possible Duplicate:
What does !! mean in ruby?
Ruby, !! operator (a/k/a the double-bang)

Sometimes I see a Ruby code like this

def sent?
  !!@sent_at
end

It seems to be not logical. Is it necessary to use here double !? As far as I'm concerned, it might be just

def sent?
  @sent_at
end

UPDATE: then what is the difference between these

def sent?
  !!@sent_at
end

def sent?
  @sent_at.nil?
end

def sent?
  @sent_at == nil
end