Only people under a certain height can see the hotline number
The ANAR Foundation in Spain is a nonprofit organization dedicated to aiding children who are abused or at risk. But faced with the problem that abusers might notice the kids looking at ads offering help, the company devised a way to make a PSA that the kids could see, but the adults couldn't.
The ads use lenticular images, which are perhaps best remembered for their use in low-quality "holograms" and "moving images" in decades past. Lenticular printing displays different images depending on the angle of view. Usually, this is done from side to side, so that walking around, or moving, the image shows you two different pictures.
In this case, ANAR calculated the average height for a 10-year old, and designed the lenticular print so that people under a certain height would see a small message and a number to call — and adults taller than that wouldn't know what was being seen. The children secretly see text that tells them "if somebody hurts you, phone us and we will help you", which will hopefully keep abusers from knowing the information that has just been imparted.
While the practicality of this ad may be in doubt (lenticular images always look a bit strange, so they might not be all that stealthy; and the ensuing media coverage has pretty much ruined the chances of this remaining a secret), it's certainly done a lot to make people talk about the issue, and broadcast ANAR's helpline far more widely than it might otherwise have been seen.
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