Rubber Sheet is the 1158th xkcd comic.
The first pane shows the narrator standing on a bowling ball which is pulling down a rubber sheet. This is reminiscent of the typical explanation for how gravity bends space and time, leading the reader to believe that that is what the narrator is explaining to the girl standing nearby on the sheet. However, in the next three panes, the ball is first pulled further down, stretching the sheet even more, then released, sending the ball and narrator flying in to the air as the sheet rebounds. Thus it is shown that the narrator was not explaining gravity, but simply having fun.
"It IS about physics. It ALL is."
The mouseover text is a response to the girl's saying "...Oh, I thought this was about physics" when she realized what was going on.