windows 10 product key, windows 10 serial number

A product key is a 25-character code that's used to activate Windows and looks like this:
PRODUCT KEY: XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
Activation helps verify that your copy of Windows hasn't been used on more PCs than the Microsoft Software License Terms allow.
How to find your product key depends on how you bought your copy of Windows 10: