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Humans hold a very accurate representation of the metrics of their body parts. Recent evidence shows that the spatial estimation of body parts length, as assessed through a bisection task, is even more accurate than that of non-corporeal extrapersonal objects (Sposito, Bolognini, Vallar, Posteraro, & Maravita (2009)). In the present paper we show that human participants estimate the mid-point of their forearm, which was kept in a radial posture, to be more distal following a 15-min training with a 60cm-long tool as compared to pre tool-use. This outcome is compatible with an increased representation of the participants’ forearm length. Control experiments show that this result was not due to a mere distal proprioceptive shift induced by tool-use, and was not replicated following the use of a 20cm-long, functionally irrelevant tool. These results strongly support the view that, although the inner knowledge of one's own …