Make:

Berlin hacker Julius von Bismarck invented and patented the Image Fulgurator, a device so awesome that it can remotely insert images into other people's photos.

You aim the device at the same subject that another person is photographing, and when they snap a photo the resulting image will be manipulated with a separate, overlayed photo. The person taking the photo will have no idea anything happened until they examine their photo.

The result is pure magic. Here's a clip of the first public "image fulguration".

The device uses a standard 35mm camera body and lens as a projector. Instead of undeveloped film, the camera is loaded with exposed, developed slide film. A flash is built into the back of the camera, sending light backwards through the body, past the the slide and out the telephoto lens. A light sensor is used to trigger the flash when another camera's flash goes off. Thus, when someone else takes a photo, the Fulgurator zaps its slide's image onto the object for a few milliseconds.

In you want to make something like this, you can use some of the techniques that folks typically use to photograph lightning. Below is a link to a simple Arduino project that will give your SLR a light activated shutter release.

While you're at it, take a crack at making your own Fulgurator with a bit more stealth factor. I'm pretty sure I'd get tazed walking around downtown waving this thing around.

It'd almost be worth it.

Image Fulgurator by Julius von Bismarck [via The Future is Awesome]
Lightning Trigger for a Camera