Input (grade 13): In conjunction with a rook, the king may make a special move called castling, in which the king moves two squares toward one of its rooks and then the rook is placed on the other side of the king.
Output (grade 5): With a rook, the king may make a special move called 'castling'. This is when the king moves two squares toward one of its rooks and then the rook is placed on the other side of the king.