Sonali Kohli wrote that adults increasingly like to spend time with colouring books:

• A colouring book is #9 on UK Amazon’s best-selling books, and has been in the top 100 for 113 days.
• There are Facebook groups for adult colouring book clubs, and Game of Thrones will soon be releasing one.
• Some think that colouring may help release stress or increase motivation, as well as recall happy childhood memories.
• Publishers have latched onto this, marketing colouring books for adults as therapeutic, to get away from any stigmas associated with adults indulging in what is usually a children’s activity.

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Source: Quartz