We analyze a dynamic second-price auction with an informed bidder and an uninformed bidder who, upon seeing a posted price, learns whether his valuation is above that price. In the essentially unique equilibrium, an informed bidder bids in the first period if her valuation is below some cutoff and bids only in the last period otherwise. An uninformed bidder bids in every period to optimally change the price unless the price is above his valuation or he is the high bidder. This model also provides a rationale behind the use of a secret reserve price in private-value settings.