Humans tend to find it strange that cats will often suddenly stop and stare in unusual directions, seemingly looking at nothing at all. What humans don't understand is that the cat is in fact not looking, but listening; the cat has heard something, and is turning its head so its ears are cupped in the direction of the sound.

Implication: If elves were real, they would often as a group swiftly turn their heads to apparently stare at nothing at all, swiveling their ears slightly as they do so

"It's kind of strange, Mr Frodo," said Sam, "You think they're lookin' at ghosts or summin'?"