Putting live video streaming in the hands of anyone with a smartphone, like Periscope did , ushered in a new era for mobile apps. But it also quickly proved that most live video is inherently boring.

HQ Trivia made live streaming far more engaging by putting three simple buttons on the screen and letting players interact in real time. And now Gravy, a live shopping game show, is taking the interactivity element in a new direction by letting a large group of viewers shop and play a game at the same time. It’s pretty addictive, too.

“We believe apps like HQ Trivia have broken open a completely new form of mobile entertainment that is going to be huge,” says Gravy cofounder Brian Wiegand. “We love HQ’s simple format of daily, live-hosted trivia. Gravy uses live show elements, but instead of trivia we are gamifying the shopping experience.”

Here’s how the app works: For each daily game airing at 8:30 p.m. ET/5:30 p.m. PT, there’s one product being sold. The product is unveiled and then viewers have 60 seconds to guess what percentage of the original price the item will sell for when the sale finishes–with Price is Right-like rules, so don’t go over.

The twist is that the quantity of the product being sold is hidden. You don’t know whether there’s one, nine, or 16 of them available. You want to wait as long as possible to get the best deal, because the discount grows as the show continues. But you can’t wait too long or it’ll sell out.

So far, Gravy has sold products like the iPhone X, AirPods, Adidas shoes, an electric skateboard, a Weber BBQ grill, Oculus Go, North Face jacket, and similar “hot” items. Generally speaking, they’re gear that’s continually in demand.