The Universe in 101 words: What is the EM drive?

Above: An ElectroMagnetic drive.

An 'EM drive' is a cone shaped, closed, conducting box. 

So far, so not exciting or strange. But fill it with microwaves, at a frequency that resonates within the box, and it feels a tiny pull. 
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That's weird: As far as anyone can tell, nothing is leaving the box, nothing is pushing on the box.... it shouldn't feel any force. Yet some NASA tests suggest this effect might be real.

Better experiments might reveal a mundane explanation. But, if that tiny force is real, a whole new kind of physics may be waiting to be discovered...