Or measuring the length of your driveway with your car’s odometer. The 50 ft driveway will always measure .1 miles or 0 miles.
With the rocks it is similar but a little different in some cases as the volcanic rocks tested were not always fully molten and had their geologic clocks reset, but were actually old rocks from the volcano that were expelled and mixed with fresh magma and thus were accurately dated as older, in my understanding of it. In any case, the dating was accurate in context given the material dated, whether by error bars or the nature of the specimen.
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