1 Young Adam: Hi Mary, Sue. We need to tell you something important.
2 Mary: I'm Mary. She's Sue.
2 Young Jamie: Uh oh...
3 Young Adam: This is terrible! We wanted to be our own grandfathers, but now we're each others' grandfathers!
4 Young Jamie: Ah, but we're our own great-great-grandfathers!
4 Young Adam: Oh, good point. Cool!

A few people have already pointed out in the forums the consequence of the discovery in this comic. I'd intended this plot development all along, so it was interesting seeing some readers predict it so far in advance.

Unless... they travelled back in time to post a "prediction" in order to make themselves seem smarter. Yes, that must be it...

I'm on to you.

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2024-07-20 Rerun commentary: Mary and Sue's names were of course chosen as a deliberate reference to the fictional self-insertion character archetype. Wikipedia's article is a little dry and technical; the one on TV Tropes is longer and more fun.