Anyone fluent in German who also knows a minimal about Werner von Braun, who designed the V1 and V2 rockets used to blitz England in 1944-45 and went on to lead the American house race, will realize the x-Stoff references as shorthand for the a variety of deadly factors of wartime German rocket fuels. Sheldon when a "terrifying" music bird flies by the condominium window and lands on his sofa space.