The Lord of the Rings: The Fellowship of the Ring (assuming Gandalf knew Frodo meant Sindarin, and not Quenya, Nandorin, Telerin or Silvan):
Frodo: It's a riddle. "Speak friend and enter." What's the Elvish word
for friend? Gandalf: Mellon. (door opens)
Don't confuse with Open Sesame, which
is when the door is opened with a spoken password.