For those of you who don't know, this is an example of the leidenfrost effect. The ball is so hot that the water touching it immediately evaporates and the vapor around it insulates it from the rest of the water (the vapor is a much worse thermal conductor than liquid water). This is why you can stick your hand in liquid nitrogen for a short period of time. However, eventually the nickel ball cools to a point where it's not hot enough to instantly vaporize the water and it stops.
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