If you like your shapes to be of constant width, friends of the Aperiodical Matt Parker and Steve Mould, who run **Maths Gear**, have long been the market leader in selling you flat 2D shapes which have the same diameter no matter which direction you measure in (well, them and the Royal Mint). But if you prefer your shapes to be of constant width in three dimensions, you can now satisfy those urges too at MathsGear.co.uk.

They’ve just launched a new product, which is a handsome set of yellow solids of constant width (for those interested, they’re not the standard Reuleaux triangle-based solid of revolution commonly sold – they’re Meissner Tetrahedra). A set of three, which allows you to test the constant width property by rolling them between a table and a book, is yours for £15, with free delivery in the UK. Tables and books sold separately.

**Buy: **Maths Gear website.