These pens work just like a normal pen, but use invisible ink. After writing your message, just flash the UV LED installed in the cover of the pen your secret message will be revealed.
ODM frequently sell these with 2 in a pack or customers will be limited to writing invisible messages only to themselves. Otherwise, you may consider having each pen packed individually so that customers will then have to buy more so that each friend can write back and view the messages with the light on their own pens (Creating a Collectible Promo).
Branding may be made with full printing on the pens. Clients can also have a printed and branded blister card for the invisible pens. Packaging can easily be customised and could also encompass pencil cases, memo pads etc…
Below we see a “Spy Pen” where the pen is branded as a fun toy for children. This could work well for promoting films or tie in with other teaser promotions.