Why doesn't my hydrant freeze?

I had never though about this until I read the "How do I Unthaw yard hydrant underground?" thread.
In my shop I have a standard yard hydrant that feeds the sink, stool, etc. The hydrant is only about 6" from the wall so the underground portion is very close to the footing. I keep the temperature in the shop at 46 degrees F unless I am out there (which is rare in the winter) and then I turn it up to about 68. In short, the hydrant is normally in an area that is 46 degrees F and it is always turned on so there is water in the standpipe at all times. Also, it might go at least a week without any flow through it (stool not flushed, sink not on, etc.) so it does not freeze because water is moving through it.
It is hard to believe that the simple fact that it is in a building with limited heat would keep the pipe underground warm enough to keep it from freezing but it has not froze in the six years since I build the shop.
Oh yeah, I live in SE Iowa so it gets down to zero sometimes (like two nights ago).
Comments welcome.
Don
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IGot2P wrote: ...

...
Well, that fact shouldn't make it _TOO_ hard to believe... :)
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How far down in the ground is the pipe, how far from the house is the shop, what is the temp of the house end, is there any insulation around the pipe that is in the ground? The pipe itself will conduct heat from its surroundings down into the ground and so if it is short enough that will keep it from freezing. What is on the ground itself. any straw or fegetation that would help keep the ground from conducting cold down? Is it a south-facing area, or a north-facing area, that will make a difference.
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IGot2P wrote:

It's probably a frost free hydrant. The water lines should be buried below the frost line. Your footing probably is too. I live in southeast Nebraska. The frost line is something like 4 feet here.
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