Time Machine
Permanent link to this comic: http://xkcd.com/716/
Image URL (for hotlinking/embedding): http://imgs.xkcd.com/comics/time_machine.png
[[Rob is working at a workbench. Future-Rob appears out of nowhere with a baseball bat.]]
Future-Rob: Hi, Rob.
Rob: Whoa, you're me!
Future-Rob: You're about to have an idea for a time machine.
Rob: I am?
[[Future-Rob hits Rob over the head with the baseball bat.]]
<<WHAM>>
[[A friend approaches Future-Rob working at the workbench. The baseball bat is stashed behind it.]]
Friend: Hey, Rob. What's up?
Future-Rob: Nothing.
This happens somewhere roughly once a month.
{{Title text: We never see any time travelers because they all discover it's a huge mistake. This is also why your friend at the lab suddenly looked about a year older recently.}}