Guilford was one of the first academic researchers who dared to conduct a study of creativity.
He challenged research subjects to connect all nine dots using just four straight lines without lifting their pencils from the page.
If you have tried solving this puzzle, you can confirm that your first attempts usually involve sketching lines inside the imaginary square.
The correct solution, however, requires you to draw lines that extend beyond the area defined by the dots.
Indeed, the concept enjoyed such strong popularity and intuitive appeal that no one bothered to check the facts.