A dramatic collision of two moons may explain why our Moon is very much two-sided. Professor Mike Merrifield briefly discusses a recent paper in Nature. Check it out at http://www.nature.com/nature/journal/v476/n7358/full/nature10289.html Visit our website at http://www.sixtysymbols.com/ We're on Facebook at http://www.facebook.com/sixtysymbols And Twitter at http://twitter.com/#!/periodicvideos