Why is the Science Museum purveying erotic pictures to children? A Post reader was taking his kids there recently when his eight-year-old spotted this intriguing painting, called Message d'amour des dauphins. It's apparently a classic optical illusion to distinguish the dirty-minded from the innocent. 'It may be a really useful test to pick out the perverts,' said the reader, who preferred not to give his name. A picture caption says adults would likely see a naked couple locked in an embrace but children would just see many dolphins: 'What you perceive here is based upon past experience. In fact, this may be the best example of priming by experience. If you are a young adult or older, you will perceive a couple in a romantic embrace. However, if you are a very young child, you will only perceive a group of dolphins. Yes! Dolphins! Can you see them?' Our reader added: ' There were a lot of children there. A whole section at the museum is devoted to optical illusions, most of which are rather more wholesome, by the way - or at least I didn't manage to spot anything naughty. I asked a museum worker there whether the dolphin picture might be a bit too much for a family-oriented exhibition full of young visitors. But she pointed to the caption, explaining children wouldn't see what I saw.'