Today I had a pair of AA batteries in my pants pocket--I don't know if the body heat started a reaction, or if they were shorted by being in contact with two pairs of keys (forming some sort of circuit?) They became so hot that I could not hold them for long, and dropped them on the grass. They continued to give off heat, so I was worried they might explode--I looked for some surface which would cool them down. The reaction did stop, after I isolated the two batteries in cool areas. Apparently about half of the charge had been discharged. I would like to know what the cause is so I can prevent this from happening again...