In short, it's because they're free. And no, this is not a scam; we repeat, this is not a scam. But let's back up a bit. When wife-and-husband duo Ellie and Quang Dinh founded their newly launched brand Girlfriend Collective earlier this year, they wanted to hit three main points: 1) to create a chic athleisure line that was minimalist in design without compromising performance, 2) to be eco-friendly and offer complete transparency with every step of the apparel-making process, and 3) to deliver a luxe product that people would trust.

How they achieved all of the above: They recruited designers, one from Acne Studios (to achieve that chic, cool-girl aesthetic) and another from Lululemon (for performance). They found a factory in Taiwan dedicated to turning plastic water bottles into recycled polyester (they break them down, melt them, and string them out into fibers). They found a fair-trade facility in Vietnam that could take the recycled polyester and weave it into garments.

"It was our unicorn fabric," Ellie tells us. "I had a lot of preconceived notions of what recycled fabric would be like, and my biggest concern was that it wouldn't feel high quality, but we were just floored—it was amazing. It's probably the softest, most luxurious fabric I have ever honestly encountered."

RELATED: The 8 Black Leggings That Make You Break Up with Pants

And, she says, there's nothing else like it on the market. It's soft, but sheer-proof. It's thick, but not uncomfortable. It is, she claims, the perfect pair of compression leggings that work for everything from lounging in to running a marathon in, on top of bettering the environment (each pair is made from about 25 water bottles). So how did these leggings go crazy-viral almost overnight? (Currently, they're backordered until January.)

"My husband actually thought of the idea," Ellie says of their marketing plan to give away leggings and have consumers pay for the shipping cost. "It's kind of scary to purchase a $100 pair of leggings from a brand you've never heard of. We wanted people to trust us, and by giving people the product, we knew they would trust us—that's how much we believed in what we're doing."

They launched in April earlier this year, spreading the word of their slightly unconventional, this-is-too-good-to-be-true free-leggings campaign through Facebook. And just like that, they sold 10,000 pairs in the first day and they received roughly 800 e-mails from consumers demanding to know if this was a scam.

"It was so nuts," she recalls. "We were just freaking out, and then the sharing feature on Facebook crashed, which helped us catch our breath for a minute."

 

This article originally appeared on InStyle.com.