They can be moody, messy and irresponsible, to the point where at times you may wonder what happened to those loving little children you once knew. Anyone who's had a grumpy teenager in the house knows this can be perhaps the toughest time to be a parent. Dr Frances Jensen is a neurologist and professor at the University of Pennsylvania Perelman School of Medicine. At times she struggled with two teenage sons, and she turned to science for some answers. And she shares those answers in a book called The Teenage Brain: A Neuroscientist's Survival Guide to Raising Adolescents and Young Adults.