works cited (apa format)
Abrahams, M. (2011, November 28). Chalk: Is it harmful to health? Retrieved November 28, 2011, from The Gaurdian: http://www.theguardian.com/education/2011/nov/28/chalk-dust-health-research
(Unknown). (2014, February 24). How was chalk first discovered? Retrieved March 6, 2014, from WikiAnswers: http://wiki.answers.com/Q/How_was_chalk_first_discovered#slide=3
Bodanis, D. ((Unknown), (Unknown) Unknown)). What is in your toothpaste? Retrieved March 6, 2014, from Hornell City Schools: http://www.hornellcityschools.com/teachers/demarco/OnlineEssays/toothpaste_bodanis.html
(Unknown). ((Unknown), (unknown) Unknown)). The Chemistary of Chalk. Retrieved March 6, 2014, from The Human Touch of Chemistary: http://humantouchofchemistry.com/the-chemistry-of-chalk.htm
(Unknown). ((Unknown), (Unknown) (Unknown)). Who invented chalk? Retrieved March 6, 2014, from Ask.com: http://www.ask.com/question/who-invented-chalk
(Unknown). ((Unknown), (Unknown) (Unknown)). What is chalk? Retrieved March 6, 2014, from Discovering Fossils: http://www.discoveringfossils.co.uk/chalk_formation_fossils.htm