At times after leaving your computer unlocked you may find your screen featuring really horrible and traumatic images or videos of pure evilness showing the lowliness of human being. These images are obviously NSFW and will be considered offensive and probably cause you puke.
Puke rainbows, that is, as said images usually display adorable kittens or puppies showing off the best of their cuteness to the world. Said images have been more likely put into your computer by Klondike. Other times you may find yourself saying kitten on IRC.
This is an internal joke but also a reminder of the importance of locking your computer when leaving it on public places as others can use it to break your security in various ways (for example leaving a trojan horse behind).
In general similar jokes are made by changing somebody's profile on social networks or inserting fullscreen offensive images or videos like those in meatspin.
Since there may be minors roaming the place and the hackerspace should be children friendly we instead choose to use cuteness and adorableness instead of offense and disgust to show the importance of securing your computer equipment.
The process to kitten somebody is as follows:
Kittening can be prevented by locking your computers (and setting a password) so others can't use it. You can also set up timeouts for locking the screen automatically to prevent others from using your computer if you have to urgently leave it unattended for some time.
In Linux you can lock your computer in most cases by pressing CTRL+ALT+L if you are using the screen utility you can lock the terminal by pressing CTRL+A then the x key.
To lock your computer with windows just press Windows+L
Ensure in your preferences → security → general you have set at most 5 second for asking the password after the screen saver starts and you have set a password for your users. Then you just need to press Control + Shift + the Eject button or, if there is no eject button then press Control + Shift + the power button this will shutdown the screen and start the screen saber.
The joke of kittening began as an internal joke after leaving some shock sites open on unlocked computers at GHS.
After putting a full screen picture of a kitten the user's computer was locked and then the prankster went to the user telling whether the user had reminded to lock his screen and cautioning the user this was going to impact him deeply.
When the scared user unlocked the computer and saw the full screened kitten image everybody in the surroundings started laughing and since the effect was less harmful whilst at the same time a good reminder than shock sites kittening kept being done.