Childish, I know, but still my favourite joke circulating unattributed on WhatsApp. Even now it still makes me laugh uncontrollably in motorway service toilets on tour.

😂 sending this to my kids.

I once watched a man pooping in a handbasin in a Heathrow toilet, so I'm not entirely sure it's a joke

I saw VIZ use this joke a few years ago. Possible clue?