Why does my NiMH battery get hot?

I currently received 2 NiMH batteries and a charger. They came with an electronic device I bought. They are appear to be all silver polished metal with no plastic casing to show a brand name. The first thing I noticed while charging is they get very hot. I researched this and found out it was quite normal. Another time after that one of the batteries got very hot in my pocket but not serious. Just today I was traveling and both of the batteries were in a loose bag together not being used. When I went to pick them out they were both VERY hot. Too hot to touch. I separated them and let em cool and discovered some white crust that may be acid or something. One of them also has a black char mark like it was burned. I have searched for some time and found nothing that says these batteries cannot touch each other. So what the heck is going on here? Are they faulty? What should I do?