At the end of a children’s variety show in Sunderland, England, organizers announced that certain ticket holders would receive a prize upon exit. Many of the estimated 1,100 children in attendance excitedly rushed toward the staircase, creating a bottleneck at the narrow exit. Those in the front were trapped and crushed, and 183 children between the ages of 3 and 14 died of asphyxiation before adults could disperse the crowd. The inquiry that followed is said to have led to what invention? Discuss