Sitting outside a bar at the weekend, I noticed something coming out of my bag that looked and also smelled a lot like a naked flame. Ordinarily, I’d have thought it was my vape, but it couldn’t have been, since that was in my mouth. So I put it down to a mirage created by the clement weather, and thought no more of it until someone much smarter than me said: “Your bag’s on fire.”

It was my remarkable good fortune to be sitting opposite a friend who is a chemistry teacher, and explained........

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