The Blue Boy is a Girl    (April Fool's Day - 1928)

The German magazine Uhu reported that x-ray analysis of Thomas Gainborough's famous work The Blue Boy (completed circa 1770) had revealed that the boy in the picture was really a girl. The x-ray analysis, done to verify the authenticity of the painting, had shown that beneath the top layer of paint Gainsborough had drawn a picture of a nude female standing in the same position. In its next issue, Uhu claimed that this spoof had been "widely believed in art circles."