One, two, three … it shouldn't be that difficult, should it? Certainly not for a master maths mentor. But in rehearsals for the BBC show Strictly Come Dancing last week, Johnny Ball got his sums all wrong as he pulled his partner, the professional dancer Aliona Vilani, in the wrong direction, twisted her leg awkwardly, and left her in hospital with a hairline fracture to her ankle. Maybe it's because they were trying an eight-count tango rather than the "one, two, three" waltz. Maybe it's just that he was determined to get fractions into the show.
While it is a dreadful shame for Vilani, who won last year's show, is it really that much of a surprise? After all, it has been more than four decades since Ball was an all-round entertainer as a Butlins Redcoat and he's been less Forsyth than Fibonacci in his career since. So what made the producers of Strictly think that, at 74 and the show's oldest ever contestant, he would be a twinkle-toed triumph?
Perhaps it was hoped that he might follow in the light tread of his broadcaster daughter Zoë Ball, who was placed third in the 2005 series. But that argument doesn't really add up. More likely they thought that, being famously eccentric, he'd be a good comedy figure, in the style of Dennis Taylor from Zoë's series.
It is a position that reached its zenith (or possibly nadir) with the infamously double-left-footed John Sergeant, who week after week drew fierce criticism from the judges and eventually felt compelled to quit the show because viewers at home were enjoying the schadenfreude just a little too much to vote him off.
But now Ball has exposed the dangers of asking a potential figure of fun to cha-cha-cha-challenge themselves to learn a complicated series of steps in a short timeframe. As Strictly judge Craig Revel Horwood might say: "That was disastrous, darling, dis-ast-er-ous". Not for Ball, mind: run with that sympathy vote, Johnny. Or maybe foxtrot with it …