Crusade in Jeans (1973) is a children's novel written by Thea Beckman. It contains a fictional account of the children's crusade of 1212, as witnessed by Rudolph Wega, a boy from the 20th century. The original Dutch title is Kruistocht in spijkerbroek.Rudolph Wega is a sixteen year-old who volunteers for an experiment with a time machine. The experiment goes well, but through an accident Rudolph is stranded in the 13th century. He saves the life of Leonardo Fibonacci, without realizing who he is, and teaches him Arabic numerals. Together they join the German Children's Crusade, and through his modern-day knowledge, Rudolph manages to save a lot of children from horrible fates. However, his knowledge also leads to accusations of witchcraft.In the book, two slavers delude a group of children into coming with them with stories of how the innocent shall liberate Jerusalem. Their actual intent is to sell them for profit. With the aid of his twentieth-century knowledge and skepticism, and the aid of a "magical" device or two (such as a box of matches), the boy manages to keep most of the children alive and eventually gets them to safety.I was drawn in by this book to a dangerous level. I was currently in Tokyo at the time and kept bumping into busy people rushing to the train because I couldn't put the book down.It captures a bit of wonder that is sometimes lost in the serious books of today. It's quite bittersweet, and you half expect the ending but then you're also kind of surprised by it too. I loved this book.