One difference between Google’s new units—which will also be able to play video clips—and some of its competitors’ ads, is that users actually have to click for the unit to grow larger; a number of other rich-media ads expand even if a user just happens to roll over them. Google (NSDQ: GOOG) also restricts the size of the expansion, so the ads won’t grow by more than twice their width or height; the idea is to generate more clicks or impressions (the ads run on both CPMs and CPCs), without disrupting the user experience too much. Publishers can opt-out of having expandable ads from certain advertisers show up on their sites using various filters.

SEE ALSO: Google Deploys Ads In News Results; Shutters Bookmarking Service

The announcement comes about a week after Google started running ads alongside Google News stories; it’s also the second policy change over the past few months that has chipped away at some of the quality standards Google had set for its ads: the company relaxed the rules for alcohol ads in the U.K. in December.