  Aligning humans' assessment of what a robot can do with its true capability
is crucial for establishing a common ground between human and robot partners
when they collaborate on a joint task. In this work, we propose an approach to
calibrate humans' estimate of a robot's reachable workspace through a small
number of demonstrations before collaboration. We develop a novel motion
planning method, REMP, which jointly optimizes the physical cost and the
expressiveness of robot motion to reveal the robot's reachability to a human
observer. Our experiments with human participants demonstrate that a short
calibration using REMP can effectively bridge the gap between what a non-expert
user thinks a robot can reach and the ground truth. We show that this
calibration procedure not only results in better user perception, but also
promotes more efficient human-robot collaborations in a subsequent joint task.
