Title text:
P.S. If you have time travel, come to my birthday party Saturday!
Transcript:
[Cueball approaches Ponytail, chiselling a rock on a shoreline next to a river with shallow rolling hills in the background]
Cueball: What are you doing?
Ponytail: This river empties onto a passive continental margin.[Cueball and Ponytail stand talking, Ponytail holding several flat rocks, in an otherwise empty and frameless panel]
Ponytail: If I chisel notes onto these rocks and throw them into the sea, they might be incorporated into some shale cliff in the distant future.[Silhouetted scene of Ponytail as she throws multiple rocks off frame to the right, Cueball watching from behind her]
[From off-panel, sound effect of a rock hitting water:] PLOP[Two 'bug-eyed aliens', sitting in personal 'hover-saucers' look rightwards at an exposed rock-face. A pick and shovel are left stuck in the ground, and one of the 'saucers' sports a mechanical arm currently holding a loose fragment of rock]
[Panel label:] 100 million years later
[Text originating from the held rock fragment:] This bedrock inspected by No. 5
Source: https://xkcd.com/3088/
The explanation page doesn’t have anything yet, but this feels like it has references to the Futurama episodes “The Late Philip J. Fry” and “Lethal Inspection”, with just a hint of Slartibartfast from The Hitchhiker’s Guide to the Galaxy
[EDIT: After making this comment, I noticed that those two Futurama episodes happen to be back-to-back with each other. xkcd must be in the middle of watching the series]