1. A computer-implemented method comprising: receiving a selection of one or more advertiser keywords, from an advertiser, to associate with an advertisement; automatically identifying one or more candidate keywords to associate with the advertisement, wherein the advertisement is associated with a cost and the one or more candidate keywords are different from the one or more advertiser keywords; associating the one or more candidate keywords and the one or more advertiser keywords with the advertisement; providing the advertisement in response to first content requests, wherein: when the one or more candidate keywords satisfy a first content request without the one or more advertiser keywords satisfying the first content request, not charging the advertiser the cost upon a presentation or selection or conversion of the advertisement and; when the one or more advertiser keywords satisfy the first content request without the one or more candidate keywords satisfying the first content request, charging the advertiser the cost upon a presentation or selection or conversion of the advertisement; monitoring performance statistics associated with the one or more candidate keywords associated with the advertisement provided in response to the first content requests; identifying a subset of the one or more candidate keywords based on the monitored performance statistics, wherein each candidate keyword in the identified subset performs better than a predetermined threshold; graduating, by one or more computers, the subset of candidate keywords for inclusion in a set of actual keywords, wherein the set of actual keywords includes at least one of the one or more advertiser keywords and at least one candidate keyword in the identified subset; associating the set of actual keywords with the advertisement; and subsequent to the graduation, providing the advertisement in response to second content requests, wherein when any keyword in the set of actual keywords satisfy a second content request, charging the advertiser the cost upon a presentation or selection or conversion of the advertisement.