What makes a great adult colouring book?
The craze for grown-ups breaking out the crayons has taken the publishing world by storm over the past year.
Amazon's top 10 best-seller list currently contains four colouring books for adults.
Scottish illustrator Johanna Basford has sold around 2m copies of her two colouring books.
As she prepares to bring out another book in the autumn, BBC News met up with Johanna and Sophie Garner a colouring fan to find out the secret of colouring success.
Some see the success of the books as a way to switch off from screens - Ms Basford calls it "digital detox" - others as a way to relieve stress.
Video journalist: Nikki Nahal. Producer: Alexandra Fouché
More video features from the BBC News Magazine
29 Jul 2015