This iconic novelty widely advertised on the back pages of comic books allows you to (apparently) see through the flesh of your hands to the bones beneath. It actually a clever optical illusion. These are plastic-framed models, not the cheaper all-cardboard versions. 
 
These are manufactured by Ming Shing, one of the pioneers of the modern joke industry, who have been creating jokes since 1963.
 
Manufacturer's safety information: Not suitable for children under 3 years. Choking hazard due to small parts. Not recommended for children under 7 years.