Welcome to the mirror. Use the blackness of your computer screen to see a reflection of you sitting front of it. Now look at your mirror image and realize there's a living being on the other side, too.
When communicating with other Wikimedians, please picture yourself in a public room filled with people. Thanks.
Original version by Spacebirdy. Success may vary with type of your computer screen.
Further reading: Incivility, Wikipedia:Civility.