Idea is interesting, I would like to see a «played» example!
Between The Lines: Collaborative Postcard Puzzle
A downloadable Postcard
There is a symmetrical picture on this postcard that can only be revealed by drawing a line through the white gaps.
By writing on the other side of this card, the person who sends it will have inscribed visual hints to help out the recipient. Hold the card up against light to see the guides and when finished drawing, cut the postcard in half using the dotted line guide on the back, swap the pieces around and join them up to reveal the full picture...
BETWEEN THE LINES!
Download, print, write and send! Colour printer required, double-sided printing recommended but not necessary, must print out to postcard size.