Taking screen shot in Android 4.0+ is simple when compared with other android versions. While capturing the animation makes it more fun. I am not sure whether this method works with all android 4.0+ versions, i am tested the method with Galaxy S2 and Nexus S and works great.
1. Press the volume down button and power button simultaneously for one second,then you will hear a capture sound with animation.
That's it ! Browse to /sdcard/Pictures/Screenshots for captured images.