Aleks notices a rainbow on our coffee table, caused by the way the afternoon light hits the beveled glass. We have a prism in the same window that fails to ever do much of anything. Stupid south-facing house. We traced the rainbow on some paper with different colored markers, then when it disappeared (oh! how short lived the rainbows are!), I got the prism down and sent them to the front door, where the only remaining good shaft of light shone. They made sparklies on the walls for quite a while.