Here's a neat trick: This Microsoft Surface table projects an image on its screen, but hold a piece of paper or glass over it, and you see a secret second layer of user interface.
The trick that Microsoft Research figured out for SecondLight make sense, when you hear it:
The screen is an LCD that can switch between opaque and transparent, alternating 60 times per second. There are two projectors underneath, each one flickering at 30 times per second. One of the projectors is timed to shine up under the screen when it's opaque, while the other is—yep, you guessed it—timed to shine through when the screen is transparent. Anything held above the screen will show what's being projected by that second projector.
The craziest thing about it is that the second projector can actually follow special objects, projecting the runner you see in the first video anywhere the panel goes, and in the appropriate proportion.
You probably noticed that the duelin' 30Hz projections make my videos wavy, but the developers said that the same thing could be done at any frame rate possible, so it's easy to imagine 60Hz and 60Hz, and not crazy to think that we might see two times 120Hz.
Microsoft's TechFest is an annual jamboree of innovation and gadgetry from Microsoft Research, which means that while none of it is coming out as is in products in the near future, it's essentially what product development people use to add cool stuff to their actual releases. I'm here all day.