How do you tune your guitar?
Do you use a pedal? A tuning fork? A pitch pipe? Or do you mentally picture "Whole Lotta Love" and use that as a reference for your low E string? (That's what I do sometimes; it doesn't always work.)
Regardless, here's a video of Guitar World gear editor Paul Riario tuning an acoustic guitar without the aid of ... anything. He actually does it twice -- once a half-step down and then again in standard tuning, all in under 60 seconds.
And yes, Paul is a show-off.