Slightly offtopic: The abacus is a fascinating tool; often considered an early prototype for tangible interfaces. "Among other historical inspirations, we sugg

Slightly offtopic: The abacus is a fascinating tool; often considered an early prototype for tangible interfaces. "Among other historical inspirations, we suggested the abacus as a compelling prototypical example. In particular, it is key to note that when viewed from the perspective of human- computer interaction (HCI), the abacus is not an “input device.” The abacus makes no distinction between “input” and “output.” Instead, the abacus beads, rods, and frame serve as manipulable physical representations of numerical values and operations. Simultaneously, these component artifacts also serve as physical controls for directly manipulating their underlying associations." (Brygg Ullmer, PhD thesis, MIT media lab)