Cat Proximity: http://xkcd.com/231/
(I think it's actually primitive-brain social intelligence, taking joy in being near the kitty, and overriding some of the usual self-censorship.)
Looking at the other xkcd's people posted, I especially laughed at "Someone is wrong on the internet." Thanks, Mage!
I would upload the images directly, but then you wouldn't get that neat-o scroll over text.
On the xkcd page, I used right-click > "View Selection Source" (or View > "View Source" or "Page Source" etc.), then copied the actual "img" tag, which contains that mouse-over title, for example:
<img src="http://imgs.xkcd.com/comics/cat_proximity.png" title="Yes you are! And you're sitting there! Hi, kitty!" alt="Cat Proximity">
Here, I clicked the "HTML" toggle button before pasting it into my message, so it goes in as an actual working tag. (I put it inside a link to the page). Clicking "HTML" again returns you to the regular writing mode.