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. 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.