body-part as object (を, not に)

Japanese marks the body part being acted upon — including the affected location of an injury or contact action — with を (direct object), even when English uses 'on,' 'in,' or a possessive: 肩をたたく ('tap on the shoulder'), 手を握る ('grip the hand'), 顔を見る ('look at the face'), 手を振る ('wave [a hand]'), 首を振る ('shake [one's head]'), 口を開ける ('open [one's mouth]'), 足をけがする ('injure one's foot'), 頭をぶつける ('bump one's head'), 手をやけどする ('burn one's hand'). The body part is treated grammatically as what receives the action.