When you place the blue and yellow sheets over something with printed text on it, you will understand why aerograms are all blue--paper of this color is less transparent, so it keeps strangers from reading your mail.
將藍紙和黃紙分別覆在文字上，就會瞭解為什麼航空信箋都用藍色紙，原來藍色紙的透明度較低，這樣我們寫在信上的秘密就不會被看透了。