Oxtra, Cristina: - Cristina Oxtra is the author of the early chapter book Tae Kwon Do Test and two biographies for children, Stephen Hawking: Get to Know the Man Behind the Theory and Stan Lee: Get to Know the Comics Creator. She earned an MFA in creative writing for children and young adults from Hamline University. Cristina currently lives in the Twin Cities (MN) area, where she and her son train and compete in Tae Kwon Do.