The Mathematician Leonhard Euler proposed that all convex polyhedrons followed a simple rule: Vertices + Faces - Edges = 2. In this investigation, children build a series of regular polyhedrons and use them to test Euler’s hypothesis. The bundle includes nets for the first five regular polyhedrons and and investigation sheet, explaining the hypothesis and providing a table for children to enter their results and space to record their conclusion. This resource was originally produced to accompany the Bodleian Libraries’ ‘Thinking 3D’ exhibition. It has been tried and tested with school groups as well as in events for the general public.