Centering items in flex is probably the most common reason for using flex. So check out this how to center items in flex vertically and horizontally which is sometimes really all you need to use flex for.
(Also see this CSS Flex Tutorial on Semicolon.)
When I started to learn Flex, I wanted to see everything it was capable of. But, I wasn’t able to find a thorough tutorial showing examples of all Flex properties. So, I created these diagrams with Flex from the bird’s eye view.