Au contraire - you don't have to use a click at all. We're humans, so we have the advantage of language. The click is merely a communication tool to let the trainee know exactly what he or she is being rewarded for. Since you and T$ have verbal language, you can just say "Thanks for remembering to shut the door! I feel much more comfortable knowing Gabe is safe." If you were training a nonverbal animal, the click (or flash of light, or hand signal, or what have you) would serve that same clarifying purpose.