1 Ophelia: {walking past Mercutio's desk} Well, I'm done for today. See you tomorrow, Mercutio.
2 Mercutio: {waving at Ophelia} Bye Ophelia. Remember, tomorrow is Causal Day.
3 {silent beat as Ophelia stops in her tracks and thinks about that for a second}
4 Ophelia: Causal Day?
4 Mercutio: Yeah. Everything we do will have effects.

I'll just wait a minute for those of you scratching your heads to reread the strip.

Okay, ready?

Causality is the principle that actions produce effects. It is important both as a philosophical principle and for a systematic understanding of science and nature.

Do not confuse it with casuality.

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2014-11-20 Rerun commentary: In retrospect, I think this strip would have been better if Mercutio had called it "Causal Friday" instead of just "Causal Day".

We don't have casual Friday at my work. Mainly because it's not unusual for some proportion of the staff to show up in jeans and T-shirts pretty much every day of the year.

Instead we have an occasional thing called "Formal Friday" where a bunch of us will come to work on a Friday wearing suits and ties. This inevitably causes confusion and amusement amongst our colleagues.