0! = 1 Dr James Grime tries to explain why this is the case - follow James on Twitter at https://twitter.com/jamesgrime Regarding the equation at the end - James says it should be e^-t dt NOT e^-n dn ... sorry for the mix-up! Website: http://www.numberphile.com/ Numberphile on Facebook: http://www.facebook.com/numberphile Numberphile tweets: https://twitter.com/numberphile Google Plus: http://bit.ly/numberGplus Tumblr: http://numberphile.tumblr.com Videos by Brady Haran Brady's other channels include: http://www.youtube.com/periodicvideos (Chemistry stuff) http://www.youtube.com/sixtysymbols (Physics and astronomy) http://www.youtube.com/numberphile (Numbers and maths) http://www.youtube.com/DeepSkyVideos (Space stuff) http://www.youtube.com/nottinghamscience (Science and behind the scenes) http://www.youtube.com/foodskey (Food science) http://www.youtube.com/BackstageScience (Big science facilities) http://www.youtube.com/favscientist (Favourite scientists) http://www.youtube.com/bibledex (Academic look at the Bible) http://www.youtube.com/wordsoftheworld (Modern language and culture) http://www.youtube.com/PhilosophyFile (Philosophy stuff) http://www.youtube.com/PsyFile (Psychology stuff)