People often visit Web sites on their mobile phones as much or more than they do on computers. But that leaves Web companies with a challenge: how to make money on phones, where there is less space for advertisements and people have less patience for them? YouTube thinks it has an answer. On Wednesday, it introduced a new kind of ad for its mobile site that lets viewers choose whether or not to watch a video ad, and only charges advertisers if the ad is watched. This type of ad, which YouTube calls TrueView, has been available on computers since late 2010. Read the full story at The New York Times blog.