There has been an increasing interest in objects within the HCI field particularly with a view to designing tangible interfaces. However, little is known about two key issues in tangible interface design: how people make sense of objects and; how objects support thinking. This paper draws upon a study of how engineers learn from mechanical products as a starting point for considering how to design interactions with physical objects that have embedded computational capabilities, that is, hardware in the tangible digital domain. The key lessons are to design devices that support interaction and user inquiry. Mechanical devices are intelligible when they: give feedback; can be reconfigured; and perform differently in different contexts. These qualities allow their users to develop inquiry processes and to gradually build a repertoire of familiar devices and operating principles.