Video:

http://www.sciencechannel.com/video-topics/space-videos/comets.htm

Sources:

http://www.qrg.northwestern.edu/projects/vss/docs/space-environment/1-what-is-a-comet.html
http://amazing-space.stsci.edu/resources/explorations/comets/lesson/facts/Fact6/index_nf.html