Why does my iPhone get so hot when I use it?

Back of phone is very warm to touch and battery runs out quickly.

All phones do that when they get hot, it's working hard and you should probably let it rest for a while so it doesn't work so hard and get so hot it shuts off.

It's usually because the phones processor is being used for a long period of time. My iPhone 4 gets extremely hot when i use FaceTime and charge it at the same time.

Could be a faulty battery as well. Just an FYI.