A square has four lines of symmetry. One line crosses through each diagonal. The other two lines cross horizontally and vertically through the middle of the square.
A line of symmetry divides an object into perfectly matching sides. Therefore, a rectangle only has two lines of symmetry because it cannot be divided diagonally to create two matching sides. A triangle, on the other hand, has three lines of symmetry, as it can be divided through each point and through the middle of the other side three times. A kite shape only has one line of symmetry--vertically down the middle of the shape.