THOSE SELF-BALANCING SCOOTERS that everyone’s calling “hoverboards” don’t actually hover. But that’s not the strangest thing about them—that would be the fact that this year’s most popular holiday gift keeps catching on fire.

An exploding two-wheeler burned down a house in Louisiana a few weeks ago; another scooter combusted in the same state in the past week. A gyroboard caused significant damage to a home in New York a few days ago. At a mall in Washington this week, a scooterboard caught fire and shoppers were forced to evacuate. The perceived danger is significant enough that major airlines have banned the little vehicles altogether.

But what is actually causing all these fires? In the New York and Louisiana incidents, the board was plugged in and recharging. In the mall incident, the board wasn’t plugged in at all; there have also been reports of scooters bursting into flames while people were riding them. Plugged in or not, the big problem has to do with the quality of the rechargeable lithium-ion batteries inside these things. They’re almost always tucked in one of the foot rests, and they work the same way as the lithium-ion batteries in our smartphones, tablets, and laptops. They’re just a lot more prone to defects.

Jay Whitacre, Professor of Materials Science & Engineering at Carnegie Mellon University, says that the problem doesn’t have to do with these self-balancing scooters themselves, but with the quality of the batteries being used. They’re cheap, and it makes sense: This is a hot (pun not intended) holiday product, the reputable models are pretty expensive, and more-affordable brands are using cheaper components to lure in shoppers that don’t want to spend a grand or more on a hands-free Segway. Predictably, a slew of cut-rate brands are flooding the market with shoddy scooters made from cheapo components.

“There are a lot of factories in China that now make Li-ion batteries, and the reality is that the quality and consistency of these batteries is typically not as good as what is found in top tier producers such as LG or Samsung,” Whitacre says. “These are known as ‘low cost li-ion batteries’ by most in the industry—they are not knockoffs or copies, but are instead just mass-manufactured cells.”

With these cheap batteries, a lot of things can cause fires. For one, the nature of a scooterboard—it can bang into stuff, smash into things at high speeds, and be abused by bros—makes the batteries susceptible to damage. It’s not just the nature of a cheap battery, it’s the nature of any lithium-ion battery. And when one of these batteries punctures, this is what happens: