This is a cool and easy way to make invisible ink and to see your writing whenever you want.
Step 1: Step 1: Materials
You will need a normal yellow highlighter, it can be some other color, but yellow is best because they usually work. I will explain later.
You will also need an ultraviolet LED, you can get some at Radio Shack for about a dollar each. A normal watch battery is also needed, obviously for lighting the LED. At Radio Shack you can get 3 for 12 dollars (big ones), I think you can get the cheapest for maybe 2 or 3 dollars each. Lastly you need a hand or two, I'm sure all of you have some of those.
Step 2: Step 2: the Important Part
I am using my hand because those are easy to find, you may use something else but you might have to dilute the ink in water for this to work. Write whatever you want on your hand or arm or something. A hand is better because it is easier to wash. You can still see the ink in the picture. Once you have written your message, go to the sink and wash it off so you can't see it. Don't worry, if you did it right it's still there, you just can't see it.
Step 3: Step 3: Almost Done!
Now just take your ultraviolet LED and put the longer lead on the + side. It should turn on and look purple-ish.
Step 4: Step 4: the Fun Part
Now you can turn off the lights. (You can see the light better with the lights off, but lights on works fine too) Shine the light onto where you wrote the message, and if you have good penmanship, you should be able to read it. (Even with my sloppy writing you can read it. It says secret message)
Step 5: Additional Information
The yellow highlighter was recommended because most are fluorescent. Fluorescence is when something reflects ultraviolet light. This is why you could see it when the UV LED was shined onto the writing. The writing wasn't perceptible in visible light because there wasn't enough of it to reflect visible light, but enough to reflect UV light. Many things are fluorescent, such as tennis balls, Astrobright paper, and even... urine. Oh yeah, if you shine it onto some shades of white, instead of purple, it reflects blue.