Eleven-month-old infants infer differences in the hardness of object surfaces from observation of penetration events.

Examples of stimuli used in Experiments 1 and 2. In Experiment 1, we manipulated three velocity changes (deceleration, constant velocity, and acceleration) involving the stick’s velocity during the “before penetration” and “penetration” phases, creating two different types of stimuli in which the surfaces of the hemispheric objects were perceived by adult observers as relatively “soft” or “crusty” (Masuda et al., 2011). Two of the three velocity changes were combined. For one of the “soft” videos, the stick accelerated before penetration and then moved at a constant speed during penetration (Soft 1). The other “soft” video involved constant velocity of the stick before penetration and deceleration during penetration (Soft 2). In contrast, for one of the “crusty” stimuli, the stick approached at constant speed and then accelerated during penetration (Crusty 1). The other “crusty” video involved deceleration of the stick before penetration followed by constant velocity during penetration (Crusty 2). In Experiment 2, the stimulus videos were identical to those used in Experiment 1, except that the gray hemisphere was deleted from the videos in the current experiment.