All you have to do is say what number is under the parked car.

For a while there, we were stumped (scroll to the bottom of the page for help with the solution).

We asked Dr David Whitebread, a senior lecturer in psychology at Cambridge University's faculty of education, why children might find it easy to crack.

"[It is a] good example of how children are much more flexible than adults in their thinking and how, sadly, we often knock this out of them in school where there is still to much emphasis on getting it right and not nearly enough on creative thinking," he told i100.co.uk via email.

























Hint: For those who still don't get it, try turning your head, or the picture, upside down.

*Update:* Several versions of the puzzle on social media say children can solve this puzzle in 20 seconds. Its creator David J. Bodycombe got in touch to say there have never been any tests to firmly establish this - and he also told us how he thinks it ended up being used in Hong Kong.