My Galaxy S was hanging, then I tried to press the buttons. That is a very normal behaviour. But while I was pressing various buttons, a strange things happened. It did a screenshot. My phone is not root yet! What have I done?! I did not know what buttons I pressed. I spent some time to try reproducing that. Then I found while I am holding the back button, and then pressing the home button. I can do the screenshot.

After that, I tried to read the operation menu, that is not such function record. But I did a google search, there are a lot of people found out this trick!