How to Reveal Lemon Juice Secret Messages in the 21st Century

If you write a message on paper using lemon juice for ink, it will be invisible when it dries.
The standard method to reveal and read the secret message was to hold the paper against a light bulb. But that was an incandescent light bulb. These days we all have LED bulbs. LED bulbs are cool to the touch. They won’t reveal secret lemon juice messages.
I’m writing a story in which the kids find a 100-year-old lemon juice secret message and they want to read it. Yes, they could use a candle or a stove burner, but I want the kids in my story to use a method that would be safe for my 10-year-old readers to use. So, I can’t have them playing with fire.
I tried a heating pad. Even when I set the heating pad on its hottest setting, lemon juice ink remains invisible.
I don’t use a hair-dryer, so my friend Jean tried it at her house. Again, the message couldn’t be read.
Yes, it is still possible to buy incandescent bulbs, if you buy the new shatter-proof ROUGH model. They cost about $4 each. But I didn’t want the kids in my story or my readers to have to go out and buy something. I wanted them to use something they could find around a typical home.
So, I tried my iron. To be honest, I buy no-wrinkle clothes and I haven’t used that iron in years, so maybe the settings aren’t accurate. But I tried it. I had to move the heat setting up to 6. (It goes to 8) before the lemon juice message became visible.
I was using an iron when I was 10 years old. But do modern kids? After all, modern playgrounds don’t have jungle gyms or teeter-totters. They don’t have gravel that can skin kids’ knees. I checked on the Internet. I found one parenting blog that talked about ironing. This blog suggested that children as young as 8-years-old could be taught to iron simple things like pillow cases. A secret message is even simpler than a pillow case.
Therefore, the kids in my story will use an iron to reveal the secret message. I’ll have the boy who helps his mother run the souvenir shop in town be the one who knows how to use it.