Write a note with lemon juice.
拿柠檬汁写一张小纸条。
It dries clear and almost invisible.
干了以后，字淡淡的，几乎看不见。
Lemon juice has sugar and acid in it.
柠檬汁里呀，有糖，也有酸。
Warm the paper near a lamp.
把纸放在暖暖的灯旁边烤一烤。
The juice turns brown and the words appear.
汁一受热就变成褐色，字冒出来了。
That is how kids made secret letters long ago.
以前的小孩，就是这么写密信的。
