How has a children’s book of eye tests been named the world’s most effective advertising campaign?

How has a children’s book of eye tests been named the world’s most effective advertising campaign? The book, Penny the Pirate, and its accompanying tablet app, were created for optometry company OPSM. In it, eye tests are disguised as illustrations, giving parents the chance to screen their kids’ vision through fun activities and skip what may be an upsetting optometrist visit. Penny the Pirate was purchased by over 126,000 parents after its release, followed by “23% growth year-on-year in the number of children's eye tests conducted by OPSM.” (Mashable)