## 2 Background and Related Work
### Tull Frameworks  
The use of physical objects to interact with digital systems has been studied and structured according to very different perspectives over time [27]. In this section we briefly review a few of the tangible interaction frameworks that have been proposed and how they relate to the design space we propose. [PERSON] et al. [28] classifies the possible associations between physical objects and digital information, according to the informational role that the objects take, as containers, tokens, and tools. Containers are generic objects that can be associated with any kind of digital information. For a container, the digital information associated with it can vary throughout the object's lifetime. Tokens are objects whose physical properties reflect the digital information associated with them. The information associated with a token typically does not change over time because it is closely tied to that particular physical object. Finally, tools are objects used to manipulate digital information. [PERSON] and [PERSON] [29] classified TUI according to how multiple objects are interpreted to form a system of physical objects. In spatial systems, the spatial configuration of the objects within some reference frame is used to define the state of the tangibles. In relational systems, \"the sequence, adjacencies, or other logical relationships between systems of multiple tangibles are mapped to computational interpretations\" [29]. Constructive systems represent tangibles that have modular elements that can be assembled together (like LEGO(tm) blocks, for example) to create different interpretations. Finally, associative tangibles represents tangibles that \"do not reference other objects to derive meaning\" [29]. [PERSON] and [PERSON] [30] proposed a \"framework that focuses on the user experience of interaction and aims to unpack the interweaving of the material/physical and the social aspects of interaction\". Their framework on physical space and social interaction for tangible interaction is composed of four themes that provide different lenses to think about the design of tangible systems. Each theme is subsequently decomposed into concepts that represent analytical tools for describing and interpreting tangible systems. Tangible manipulation refers to the bodily interaction with physical objects that are somehow coupled with computational functions. This theme is concerned with the material qualities of the objects and how their manipulation is associated with the effects on the system. Concepts in tangible manipulation are: haptic direct manipulation (ability to grab, feel, and move), lightweight interaction (ability to interact in small steps with immediate feedback), isomorph effects (ability to understand the relation between actions and their effects). Spatial interaction refers to how tangible interaction is embedded in physical space and requires (bodily) movement in space. Spatial interaction includes concepts such as: inhabited space (whether people and objects meet and form a meaningful place), configurable materials (space is configurable and appropriately, moving things around has meaning), non-fragmented visibility (whether everybody can see what is happening), full-body interaction (whether we can use our whole body), performative action (whether our body movements can communicate something while we are doing what we do). Embodied facilitation refers to how the physical configuration of objects in space directs group behaviour. Concepts in embodied facilitation are: embodied constraints (whether the physical configuration constrains behavior leading to collaboration), multiple access points (whether all users can see what is happening and participate by using the central objects), tailored representations (whether the representations are familiar and connect with users' skills). Expressive representation focuses on what tangibles represent, how expressive they are, what material and digital representations are employed, and how they are perceived by users. [PERSON] et al.