I prepared this ppt as a warm-up for a lesson about extreme sports. It can be used as a 'back to board' game or as pre-teaching vocabulary. The 'back to board' game goes like this: the class is divided into teams (2, 3 or 4). One student comes and sits on a chair with his-her back to the board. His-her teammates have to explain the word so that he-she can guess it. Teams take turn, each team gets the same number of words to guess. My students love this kind of game. It works with other vocabulary, as warm-up or revision.