This is a feature I use once in a while to grab screenshots for my blog or to share with friends. On any Android device, hold down the volume down and Power buttons at the same time (for about half a second) and you will see a little graphic showing you a screenshot has been taken.

At first you may have some difficulty finding the right wait time but eventually this will become second nature.

Screenshots [on any Nexus device] are saved in:

/storage/sdcard0/Pictures/Screenshots