Once I needed to take a screenshot on my Android phone (Nexus 5) without installing any application. I found that Android has integrated function/option for capturing/taking screenshots of any application or screen environment on your phone. You just need to press and hold power + volume-down buttons together for 1-3 seconds, then you will see a notification on the phone’s status bar with «Saving screenshot…» icon and a new folder «Screenshots» will be created in the phone’s gallery (where all your screenshots will be stored).
If you see «Screenshot can't be saved» notification, it may be caused by lack of internal storage space or by incorrect settings for saving pictures. You need to check your settings for saving images or clear internal storage (e.g. delete some images, files or applications).
This integrated function works on Android 4.3, 4.4, 4.4.1, 4.4.2, 4.4.4 KitKat, 5, 5.1, 5.1.1 Lollipop, 6.0 Marshmallow.
If you have Samsung phone and power + volume-down buttons don't work, you can try to press and hold power + home buttons together for 1-3 seconds.
Warning! If you hold buttons more than 1-3 seconds, your device will power off.