I guess I'm having summer brain, because I suddenly realized it's the third Monday of the month already, and I haven't written my post. So, this proves that it's not just kids who get summer brain. Parents do too.
I don't know about your kids, but mine love secrets and codes and maps. I've been seeing several posts this past week about using homemade litmus paper for art projects and creating treasure maps and other things. Well, I have a new plan to make a treasure map using invisible ink.
Supplies: lemon juice, computer paper, q-tip or narrow paint brush
First gather a large amount of lemon juice. I haven't tried the concentrated lemon juice yet, so I can't say for sure if it works just as well. But, if you're making this with the kids squeezing the juice out is a great hand exercise and it helps the kids with their fine motor control.
Create your map or secret message. A tip I learned, don't let your paper get totally saturated, it has a high chance of ripping then, also it doesn't seem to reveal as well.
When you are revealing the message you can use a candle or a stove burner. Both obviously require adult supervision. The book we read that first inspired us to try it claimed you could reveal it by breathing on it, but our experiments did not cause this to happen.
Extensions beyond a fun activity:
Look at the science behind it (this also has suggestions of other invisible inks to try)
Find out about invisible inks in history, the part I found interesting is the claim that they are still in use by certain government agencies as recently as a few years ago.
Read about the use of codes in history. I find the Navajo Code talkers fascinating.