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![](http://upload.wikimedia.org/wikipedia/commons/thumb/4/42/Dermatographic_urticaria.jpg/350px-Dermatographic_urticaria.jpg)
Dermatographic urticaria ("skin writing"). Done by writing the "invisible" letters with an empty ball-point pen and waiting for a couple of minutes.
Photo credit: Mysid (public domain)
Photo credit: Mysid (public domain)