By Alex Fitzpatrick
September 9, 2016

Samsung announced last week that it’s recalling millions of its new Galaxy Note 7 smartphones amid reports the devices were overheating and, in some cases, catching fire.

There haven’t been any known cases of people being physically hurt by the phones. But the devices are being blamed for fires that destroyed a Jeep and did serious damage to a garage. Meanwhile, U.S. aviation officials are now telling fliers to avoid turning the phones on in flight.

The South Korean electronics manufacturer has yet to explain exactly what’s causing the fires. What we do know, however, is that the problem stems from their battery.

Like almost all modern smartphones and lots of other consumer electronics, the Note 7 uses a rechargeable lithium-ion battery. Electronics companies favor these kinds of batteries because they’re cheap, they pack plenty of power, and they don’t lose a lot of their charge over time when they’re sitting idle. Yet they have also been involved in several high-profile fire incidents, including episodes that more or less ended the hoverboard craze and caused problems with Boeing’s 787 jetliner.

How do lithium-ion batteries work, and why do they seem to be prone to catching fire? We asked Dr. Donald R. Sadoway, the John F. Elliott Professor of Materials Chemistry at the Massachusetts Institute of Technology (and onetime TIME Most Influential Person), to explain. First, he offered a quick primer on battery chemistry:

 

Sadoway then brought up the battery fires that plagued Sony laptops nearly a decade ago:

Right now, it’s hard to tell exactly what’s causing Samsung’s problems. But Sadoway has some theories:

So does Sadoway think lithium-ion batteries are still safe to use in smartphones and other electronics?

Write to Alex Fitzpatrick at alex.fitzpatrick@time.com.

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