Review on Unlocking Intrigue: Geddes Top Secret Spy Assortment Unveiled by Jonathan Yang

Great idea! Used at a children's spy party. The kids loved them, I bet some markers didn't show what was written in invisible ink. I'm not sure if the problem is with the pen or marker. I've tried all the problematic pens myself to make sure it's not just kids who write sloppily, but they still don't work.