Imagine a way in which you could use the same pen to draw a black line on a white surface and a white line on a black surface without knowing what color the surface is.

• 想象一下你不用知道表面的颜色是什么，就可以用相同的画笔在白色的表面上绘制一条黑色的线并且在黑色的表面上绘制一条白色的线。