freethink
Explanation
This is a promotional/meta comic rather than a traditional joke comic. Zach Weinersmith (the creator of SMBC) uses the comic format to promote a Freethink video appearance.
In the top panel, a cartoon version of Zach announces: "Hey, me and Kelly are in a Freethink video, talking about space settlement. Click to see!"
Below is a screenshot/thumbnail from the actual Freethink video, showing what appears to be Zach being interviewed on camera for the "Hard Reset" series.
The bottom caption reads: "We could talk about poop though."
The humor, such as it is, comes from the contrast between the serious topic (space settlement -- Zach and Kelly Weinersmith wrote the book "A City on Mars" about the challenges of space colonization) and the irreverent punchline suggesting they'd rather discuss something crude. This is typical of Weinersmith's self-deprecating style: promoting serious intellectual work while undercutting it with bathroom humor. The comic is essentially an ad for the Freethink video, dressed up with a small joke to keep it in the spirit of the daily strip.