1 Mary: What are you two blathering about?
2 Young Jamie: I guess it's time to come clean. We're actually from the future. We're your grandsons.
3 Sue: You mean... we... with our own grandsons??
3 Mary: Yuk!!
4 Young Adam: Actually, we messed up. With each other's grandson.
4 Mary: Oh. Well, that makes it much less creepy...

I'd actually planned this "each other's grandsons" thing well in advance. I had no idea how many readers might guess it before it was revealed (in the previous strip in this story arc (which is the second-previous strip in this theme)). As it turns out, as I write this, a couple of people on the forums have suggested it. So kudos* to you!

* Which is not a plural, for anyone not aware of that.

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2024-08-18 Rerun commentary: More cool English words which are sometimes mistaken for plurals (don't leave the 's' off, folks!):

• biceps
• crossroads
• headquarters
• series
• species
• grandsons