Dear Lazyweb:

**ghoti** and I were trying to explain a strange shape to

**xanna** and Jacob in the hope that they knew the proper geometric name for it. This is the shape of the containers that the ice lollies we like come in (

old but still accurate picture, search for "Jubbly"). It's the original tetrahedron shape used by Tetra Pak, now called the Tetra Classic (indeed the pack is so labelled). It's not a regular tetrahedron, though: it's constructed from four isosceles triangles. It's most easily constructed using this net (apologies for the dodgy quality of my Inkscape use):

Fold it such that edges A meet; in doing so edges B will also meet, along with edges C.

MathWorld reckons that it's a special case of an

isosceles tetrahedron, but that only requires opposite pairs of the tetrahedral edges with the same length as A to have the same length, whereas in fact four edges have the same length in this shape.

Does anyone know if there's a proper name for this polyhedron? Doubly-isosceles tetrahedron or something?

alison_leesWarning: I'm not a mathematician, and I think maths is boring...

cartesiandaemon