神奇的纸和笔

星期五下午，我看到一个同学有一种不一样的纸和笔。这种纸和笔可真神奇，它神奇在哪里呢？让我来告诉你吧！这支笔的笔毛是白色的，可是在一种纸上写字，写出来的字却是黑色的。这种纸也很神奇，就是写出来的字一会儿就自动消失了。这是为什么呢？

On Friday afternoon, I saw a classmate with a different kind of paper and pen. This kind of paper and pen is amazing. Where is the magic? Let me tell you! The brush of this pen is white, but the writing on a kind of paper is black. This kind of paper is also very magical, that is, the words written will disappear automatically in a short time. Why is that?