How to take a screenshot on Android

How do I take a screenshot on my Android device (e.g. take a screenshot on smartphone or take a screenshot on tablet)?
hold down at once	Volume Down	and	Power	for 1–2 seconds	=	The screen flashes white. Your device captures the entire screen and saves it as a photo.
Where do I find my screenshots?
Check to see if your Gallery or Photos app has a Screenshots folder.
I see no screenshots. Is there something else I could try?
hold down at once	Home	and	Power	for 1–2 seconds	=	The screen flashes white. Your device captures the entire screen and saves it as a photo.
I'm still unable to take screenshots. Why?
On most devices you need Android 4.0 or later.