Design documentation does not typically include all of the information sought by mechanical design engineers. This paper reports on a study of practicing engineers making modifications to existing designs. Particular attention is paid to the design information required to answer questions about the design and to verify or refute conjectures about the design. A taxonomy of the questions asked by the designers in this study and the conjectures they formed is presented. It is proposed that an intelligent CAD system be developed to capture, structure, and re-play this information.