I would say: You may think philosophy is stupid, but at least I'll expand my mind more than you ever could in a single life time. I'll question why things are the way are and gain more knowledge and it will not be useless. Not everyone has to be practical in the sense that they are directly contributing to society. The people who take on degrees such as nursing that directly contribute to society are not better than the people who indirectly help society. Sometimes the people who indirectly help society have a greater impact. "One day my thoughts could go into history and I could completely change everyones perspective of the world." That's pretty significant to me.