We kind of figured that perhaps invisible ink was used to smuggle messages. (Don't ruin my enthusiasm if I'm wrong!!)
So today we made messages with invisible ink and I asked the girls to figure out how it worked. And, of miracles of homeschooling, they did!
They used lemon juice and a solution of half baking soda, half water to write their messages. When I asked what they noticed about their choices, they realized they were both acids.
As they applied the solutions to the paper, I asked what they thought was happening. They knew that acid wears away your teeth so they figured that the acid was wearing away the paper.
After the messages had dried, we heated them over a flame to reveal the writing. They decided that the part of the paper that had been worn away by the acid burned faster and revealed the message.