Question: How many 18-wheelers would be necessary to contain the entire quantity of pure heroin consumed in the U.S. every year?

Answer: One. The heroin would fill one-sixth of the available cargo space.

[1] YRC. (2016). "Trailer Dimensions." YRC Worldwide website. Accessed February 26, 2016, at http://yrc.com/trailer-dimensions/
[2] RAND Corporation. (2014). "What America's Users Spend on Illegal Drugs: 2000-2010." Office of National Drug Control Policy website. Accessed February 26, 2016, at https://www.whitehouse.gov/sites/default/files/ondcp/policy-and-research/wausid_results_report.pdf
[3] Lookchem. (2016). "Heroin." Lookchem.com website. Accessed February 26, 2016, at http://www.lookchem.com/Heroin/
[4] RAND Corporation. (2014). "What America's Users Spend on Illegal Drugs: 2000-2010." Office of National Drug Control Policy website. Accessed February 26, 2016, at https://www.whitehouse.gov/sites/default/files/ondcp/policy-and-research/wausid_results_report.pdf

Question: How much is a child's backpack full of heroin worth, once the heroin is divided up and sold on the street?

Answer: Roughly $15 million.

[1] Parker-Pope, Tara. (2009). "Weighing School Backpacks." New York Times. July 21. Accessed March 7, 2016 at http://well.blogs.nytimes.com/2009/07/21/weighing-school-backpacks/?_r=0
[2] RAND Corporation. (2014). "What America's Users Spend on Illegal Drugs: 2000-2010." Office of National Drug Control Policy website. Accessed February 26, 2016, at https://www.whitehouse.gov/sites/default/files/ondcp/policy-and-research/wausid_results_report.pdf