Wolf Elective 1B is to use a secret code using Invisible Ink. I tested the methods in the Wolf book prior to teaching them to the boys. The problem with using Milk or Lemon Juice as your invisible ink, is that you need a light bulb to "burn" the message. Many people are switching to florecent lightbulbs and they don't produce enough heat to burn the message. In fact, I even tried heating the message up over the stove, and didn't have much luck. Not to mention, I thought better about the boys using a heat source.
I found an alternate invisible ink source. You can use baking soda and water as your "ink". 1 part baking soda to 2 parts water. The baking soda and water don't mix well, so you will need to stir with each dip in your "ink". To make your message appear, use concentrated grape juice. The grape juice reacts with the baking soda to create a chemical reaction and turns the secret message a different color. This works great and you don't have to worry about your cubs accidentally burning the house down.
You can use Q tips to apply both the secret message and the grape juice.
Note: Don't let the boys get carried away with the grape juice. The wetter it gets, the more the message smears. But if they do a quick swipe with the juice, the message appears with no problems.