(NBC) -- It can be difficult to reach out to mistreated children, since their persecutors are likely to prevent them from hearing or reading anything that might let them escape or alert the authorities. This street advertisement uses a clever optical trick to make emergency information visible only from a kid's perspective.

The sign is a concept for a Spanish organization called Aid to Children and Adolescents at Risk, otherwise known by its initials in Spanish, ANAR. The idea is to have an ad that looks, to adults, like an ordinary awareness campaign about child abuse — but to show emergency contact information only to kids.

To do this, design firm Grey Group used a technique called lenticular lensing. It uses a layer of thin, angled lenses to refract a specially-printed image into a number of separate images, each of which is only visible from a certain angle or set of angles.

Lenticular lenses and displays have been used many times before, from advertising to album art — chances are you've seen one recently. The Nintendo 3DS handheld, for example, uses it to send one image to the left eye and another to the right, creating the 3-D effect. But this is the first time that it's been used to direct a hidden image visible only to kids, who would view it from a different angle than adults.

As you can see in the video, the image of the child in the picture has injuries when viewed from below, and a message appears next to an easily-memorized phone number. It reads, when translated: "If somebody hurts you, phone us and we'll help you."

The effectiveness of such an ad campaign is another discussion, but it's certainly a good example of technology like this being used in an interesting and constructive way. More information on the campaign can be found at ANAR's website (in Spanish).

Devin Coldewey is a contributing writer for NBC News Digital. His personal website is coldewey.cc.