Take a look at this picture of an old bottle. What image do you see in it?
If it's a couple getting frisky, then congratulations - the likelihood is that you're a fully-grown adult.
If it's nine dolphins frolicking around, then you're either very young, or you need to get out more.
The internet is full of so-called optical illusions, but this one – revived in various corners of the web recently and often cited as a 'how dirty is your mind?' test – is actually what you might call Proper Art.
It was created by Swiss painter Sandro Del-Prete who, a bit like M. C. Escher (the hand drawing itself guy) plays with the different between 'looking' (a first glance) and 'seeing' (things being fully processed by the mind).
The theory is that children and other uncorrupted innocents are incapable of seeing the sexual scene and can only see the sea life, while the rest of us jaded perverts immediately see something rude.