In the last few weeks I have both moved into a new house and constructed hexaflexagons in celebration of Martin Gardner’s birthday. If you don’t know what I’m talking about then you *must* go and watch Vi Hart’s videos on the exciting topic. This unusual combination of activities led me to discover a remarkable Ikea-hack: the flat-pack hexaflexagon. Excitingly, this product is available at all Ikea stores completely free of charge! It is cleverly disguised as an appallingly *practical* paper measuring tape, but a small amount of construction will transform the strip into an entertaining mathemagical toy.

Pi is wrong. This startling assertion became so abundantly clear to me last week that I was surprised it has taken this long for me to encounter the arguments. Of course, as the number of diameters in a circle’s circumference then pi = 3.141592 653589 793238 462643 383279 50288… is technically correct. But it is wrong *conceptually*! Mathematically it is the radius, not the diameter, which is the defining dimension of a circle.

And so a better circle constant is τ (tau) =2π. Happily I learnt of this just in time to celebrate Tau Day.

There are many excellent reasons why tau is better than pi, and I won’t bother presenting them all here. Michael Hartl makes the argument convincingly in The Tau Manifesto if you are looking for some reading, but this video presents the salient points in a thoroughly entertaining way.

If you’re strugging to catch on to tau as the new circle constant, then maybe this musical representation of the number will help.

We dutifully and cheerfully celebrated Tau Day with two pies, and it is a shame that next June 28 is a year away. I guess that is a full year in which to extol the virtues of Tau.

And there’s another job to do. The 36 digits of pi listed above were written from memory. I have them firmly lodged in my brain as a result of a friendly competition in grade 5. Now I’m going to have to memorise at least 37 digits of tau!