1 [sound]: Knock! Knock!
2 Charity Collector Guy: Hi. I'm collecting to fund fundamental research into time travel.
3 Steve: Crikey! I know! You were here yesterday too!
4 Charity Collector Guy: Not yet.

I was astonished to discover that the Charity Collector Guy and Steve appearing together - something I'd come to think of as one of the standard gag set-up formats in Irregular Webcomic! - hasn't happened since #978, almost two years ago.

* I will actually get this idea courtesy of Ian Stewart and Jack Cohen, in The Science of Discworld III: Darwin's Watch. But it will be so cool that I have to repeat it somewhere.

Time travel is a very interesting subject. Which I don't have time to go into here.

Okay, now I do. Which is where stuff happens before the thing that should have led up to it.

It'd be nice if time travel existed, because it leads to all sorts of cool stories and stuff, but unfortunately it also leads to stuff like causality paradoxes.

Personally, I think if you manage to go back in time and successfully kill your own grandfather, then you've pretty much guaranteed that he turns out to be a vampire, and rises from the grave the day after you go home. Then you end up wondering where you got the sudden liking for plunging necklines and aversion to mirrors.*