Plug in the AAA battery holder to the Gemma M0 and turn on the battery pack's power switch. Be sure not to touch the capacitive pins on the Gemma M0 while it is starting up, as there is a capacitive touch calibration that runs to get a baseline value.
Once it has started, touch the D0 pad and you'll see the LED light up!
Use your UV marking pen to write a secret message on a piece of paper, or page of a newspaper or book. The naked eye won't see any indications of the writing. Now you can send the message along to a compatriot, and nobody snooping around will be able to see your important intel...
Especially if you turn out the lights first! This composite photo shows the full message at one time -- you'll need to reveal each letter one by one, unless you and your fellow spies have very small handwriting!
Unlike the old lemon juice and light bulb trick -- the marker ink isn't "developed" it fluoresces under UV light -- if you hear someone coming, lift your finger off the cap touch pad and the message will disappear!