A hallmark of the traditional approach to teaching is the importance of practice. Computer aided assessment provides one mechanism to automate this, providing immediate feedback to students and relieving teachers from repetitive marking. This paper is a follow-up to the JEM presentation given in Lisbon in Feb 2007 in which we asked "what is a mathematical question?". In this talk we will provide our answer. In particular we shall demonstrate the concept of mathematical question which has been implemented as part of the STACK computer aided assessment system. This include multiple "parts" which may involve interactions of a number of kinds. These may be independent or linked, and may implement "follow through marking", where appropriate. STACK questions are used by students through the Moodle content management system.