Write a question about the following article: Reid and Saajid Badat, another British man preparing as a terrorist, returned to Pakistan in November 2001, and reportedly travelled overland to Afghanistan. They were given "shoe bombs", casual footwear adapted to be covertly smuggled onto aircraft before being used to destroy them. Later forensic analysis of both bombs showed that they contained the same plastic explosive and that the respective lengths of detonator cord had come from the same batch: the cut mark on Badat's cord exactly matched that on Reid's. The pair returned separately to the United Kingdom in early December 2001. Reid went to Belgium for 10 days before catching a train to Paris on 16 December. On 21 December 2001, Reid attempted to board a flight from Paris to Miami, Florida. His boarding was delayed because his dishevelled physical appearance aroused the suspicions of the airline passenger screeners. In addition, Reid did not answer all of their questions, and had not checked any luggage for the transatlantic flight. Additional screening by the French National Police resulted in Reid's being re-issued a ticket for a flight on the following day. He returned to the Paris airport on 22 December 2001, and boarded American Airlines Flight 63 from Paris to Miami, wearing his special shoes packed with plastic explosives in their hollowed-out bottoms.

Question about the article:
What material were the shoe explosives made out of?