On 1/8/2013 5:35 PM, James Propp wrote: Does anyone know which sort of rollers the MoMath exhibit "Coaster Rollers" uses? Jim Propp GeorgeHart>Jim, There are four shapes of rollers in the MoMath ride-on exhibit. Three are acorn-like surfaces of revolution and the fourth is a Meissner tetrahedron. One of them is very similar to this very cool commercially available metal set: http://www.grand-illusions.com/acatalog/Solids_of_Constant_Width.html The second has a point at the apex but is differentiable everywhere else, and the third is based on a regular pentagon (extended with radii and rotated to a surface of revolution). (I also wanted to have one sphere in the mix, with a different color, to emphasize that all the others aren't spheres, but that was vetoed...) George http://georgehart.com/ Too bad. Viewing the rollers through the transparent floor should reveal the ball moving more slowly due to its greater circumference. It must be a bear keeping that floor descratched. You probably want a Gorilla Glass sandwich. --rwg