This is tricky to explain: Imagine that one of the pixels on your monitor could jump right off of the screen, and float in front of your face. Just one pixel wouldn't be much use for conveying information. But imagine if that pixel could then zig and zag through the air, fast enough that it could draw letters or images you can read.

That's essentially what researchers at the UK's University of Sussex have managed to accomplish. Their "pixel" is actually a 2mm bead of polystyrene that they are able to float, and precisely manipulate through 3D space, with the use of ultrasound. It can then "draw," and it moves so fast that it does not flicker, as it does when it appears on camera:

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The research paper is called "A volumetric display for visual, tactile and audio presentation using acoustic trapping" and is freely available.