A training ship is a ship used to train students as sailors. The term is especially used for ships employed by navies to train future officers. Essentially there are two types: those used for training at sea and old hulks used to house classrooms. The hands-on aspect provided by sail training has also been used as a platform for everything from semesters-at-sea for undergraduate oceanography and biology students, marine science and physical science for high school students, and character building for at-risk youths.