This photo shows the contents of one of my pockets. I went out on a service call for a furnace yesterday and on my way out of the shop I grabbed a couple AA batteries for my flash light. I was in a hurry so I just tossed them in my pocket.
A few hours later I felt something hot on my leg. At first I thought maybe I had been leaning against one of the hot pipes in the boiler room. Then I looked down at my leg thinking that maybe I had spilled some of the hot glycol from the boiler onto myself. When I saw that my pants were dry I realized that the heat was coming from my pocket.
When I reached into my pocket one of the batteries was very hot. Not hot enough to burn my fingers, but hot enough to make me think about it. Apparently the battery was shorting out on all the metal stuff in my pocket. From now on I am going to be more careful with batteries. I don't know if they could get hot enough to start a fire, but I don't want to find out.