The Djeco Caroline Magic Pen is a felt-tip pen with invisible ink that only becomes visible when exposed to ultraviolet light emitted from the cap. It operates on included batteries. The child writes or draws normally, and the line remains hidden until revealed by the UV light, transforming each message or drawing into a little secret. The pen body features an illustration of Caroline, one of the characters from the Djeco Magic Pens collection. It's a toy suitable for children aged 8 and up, popular for exchanging secret notes between friends or siblings.