Gabriel Weston explores the forensic advances that have elevated murder weapons from mere instruments to crucial pieces of evidence. She sees how arsenic, the undetectable weapon of choice in the 19th century, was exposed as the killer thanks to one simple chemical test, and finds out how distinctive marks on a victim's skull led to the weapon and, in turn, the killer. Gabriel also looks at the latest forensic developments, from 3-D laser scanning that can reconstruct the sequence of events at the scene of a gun crime to the pioneering chemistry able to determine where in the world someone has been based on just a few strands of their hair