After 35 reported incidents of overheating smartphones worldwide, Samsung made the unprecedented decision to recall every single one of the Galaxy Note 7 smartphones sold.
If the battery short-circuits -- say, by puncturing the incredibly thin sheet of plastic separating the positive and negative sides of the battery -- the puncture point becomes the path of least resistance for electricity to flow.
It heats up the (flammable!) liquid electrolyte at that spot. And if the liquid heats up quickly enough, the battery can explode
the thin plastic layer that separates the positive and negative sides of the battery got punctured, became the shortest route for electricity to zap across the battery (that's why they call it a "short-circuit"), and became a huge fire risk.
Phones use lithium ion battery packs for their power, and it just so happens that the liquid swimming around inside most lithium ion batteries is highly flammable.
If the battery short-circuits -- say, by puncturing the incredibly thin sheet of plastic separating the positive and negative sides of the battery -- the puncture point becomes the path of least resistance for electricity to flow.