After writing with the pen, the ink will not be visible. You need to turn on the purple light on the cap to illuminate the writing area, so that you can see the confidential information clearly and perfectly.Harmless and odorless ink, a healthy toy for your child. Smooth, colorful disappearing ink on T-shirts, paper, skin and any surface. Permanent but washable.Write your blessings on the gifts for friends to make the party more interesting. Use the lights on the pen caps to make the words appear so fun and amazing, birthday gifts, halloween gifts, children's Christmas Day gifts.Custom logo is available.Please call for more details.