Now that robots are leaving their cages in factory shop floors and laboratories, they are confronted with human everyday worlds. With this transfer from being exclusively concerned with technical systems to building socio-technical systems, everyday worlds become a wicked problem of robotics: They are interpretative, highly context dependent and products of constant interactive negotiation. The key to understanding and modelling this wicked problem is acknowledging the complexity of a human interaction. We stress three basic factors, which are constitutive for this complexity: indexicality, reciprocity of expectations, and double contingency. Although it is in the nature of these factors that they cannot be formalized, roboticists are forced to translate them into complicated, rather than complex, formalizations.