A good brainteaser is hard to find

I enjoy a good brainteaser, one that you really have to concentrate on and with enough revelations built in that make the end result a satisfying accomplishment. Here are some of my favorites. I made the answer text white so that you can’t see it unless you highlight (click and drag) over it.

Question: If you place 3 points randomly on the perimeter of a circle, what is the probability that all 3 lie on the same semi-circle?

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