Take a closer look at this photo.
Six friends on a sofa, but only five pairs of legs.
In what threatens to be the next optical illusion viral hit (remember The Dress people were talking about for six months?), this photo only gets more confusing the longer you look at it.
It was was posted on Reddit by user 'jr0d7771' on Christmas Eve with the caption "Find the middle girl's legs" and has attracted over 400 incredulous responses – and a few unconvincing stabs at clearing things up – since.
Perhaps it's the brie and sherry hangover, but at the time of writing we're no closer to figuring it out ourselves – unless the girl in question is 'manspreading' and the two either side of her only have one leg each, which somehow seems unlikely.
Answers on a postcard, please.