Among the studies dedicated to design creativity, a significant attention is given to the investigation of its dimensions, such as novelty and usefulness. The underlying assumption is that an enhanced knowledge of them is helpful to better understand limitations of current design approaches, and improve methods and tools. While there is still a lively discussion about these dimensions, some authors highlight that among them surprise deserves to be considered an independent aspect that differs from novelty. In fact, the latter concerns unprecedented peculiarities of an artefact, while surprise tells about the unexpectedness of a feature whatever is the degree of difference with pre-existing ones. Having observed the lack of reference models to investigate the emergence of surprise when a user first meets a new artefact, the authors propose an original model to describe the occurring cognitive processes. The model exploits some fundamental concepts of Gero s situated FBS framework and represent surprise as a mismatch between the interpretation of reality given by an observer and her/his expectations due to previous experiences. The model is illustrated by means of three examples.