The various segments of the top triangle are rearranged to form the new triangle below which is exactly the same dimensions, and yet it is missing a square — it has been taken away.
It’s an optical illusion, of course. That is what the programme was about. The outer line formed by the red and blue triangles isn’t actually straight in the top version, but is slightly bowed. If you want to learn more about how amateur mathematician, Paul Curry, devised this, try this site.
The programme also highlighted research that has shown that it isn’t striped clothes that make you look thinner, but hooped ones. Paul Thompson from York University who carried out the study was in the audience to defend his findings against a belligerent Johnny Vegas on the panel.
As you might expect, the Daily Mail leapt upon this story on page five the following day with photos of Kelly Brook in a red and white vertical striped frock and Hayden Panettiere in a black and white hooped number to illustrate the point.
The thing is, the QI programme was a repeat. I remember seeing it on Dave months ago so the Mail’s news wasn’t news at all. And they slag off the BBC for showing repeats.