In a sealed-bid auction where the winner pays the SECOND-highest bid, the smartest move is to bid EXACTLY what the thing is worth to you — no bluffing, no shading. Overbid and you might win at a loss; underbid and you might lose a good deal; truth is the dominant strategy. That elegant trick (Vickrey, 1961) is how Google sells ad slots. Slide your bid and see truth win.
In a Vickrey (second-price sealed-bid) auction the highest bidder wins but pays the SECOND-highest bid. This makes truthful bidding a DOMINANT strategy: your bid only decides WHETHER you win, never how much you pay, so you should bid your true value v — bidding above risks winning at a price above v (a loss), bidding below risks losing a profitable auction. Truth-telling maximises your surplus (v − second price) no matter what others do. A fail-loud self-check throws unless the winner pays the second price and no deviation from the true value improves the outcome. ◆ real mechanism design, node-verified.
A single-item private-value Vickrey auction (the exact truthfulness result); real ad auctions add reserve prices, budgets and correlated values (and generalised second-price differs subtly) — the pay-the-second-price-makes-honesty-dominant mechanism is exact.