When a building burst into flames in New York last week, bystanders filmed the blaze on their smartphones, sharing the footage online in real time. Live-video streaming is the latest hot technology trend, spreading thanks to two startups: Periscope and Meerkat. Just as Instagram popularised the sharing of photographs online, live-video streaming is poised to become an increasingly important cultural force. Twitter is banking on its becoming a commercial opportunity, too: in January it bought Periscope, reportedly for $100m. Certainly, the technology’s impact on diverse industries, such as music and journalism, will be profound, as people share their experiences at concerts and news outlets use eye-witness clips in their reports. Whether Meerkat and Periscope will become big businesses—and Twitter’s bet pays off—is less clear: advertisers are likely to be cautious about placing ads in live streams. And like so much of the internet, the apps are entirely free.