Suppose, in repaving your driveway, you find a stash of old coins buried in the ground. Of course there are more outlandish explanations, like somebody counterfeiting 1920 coins in 1900 (and successfully anticipating any changes in design in the meantime), or secretly tearing up part of the driveway after 1950, but unless someone comes up with really persuasive evidence, we're justified in ignoring these hypotheses.
Minerals used in isotopic dating
Potassium-argon dating is very susceptible to resetting because the argon decay products are merely held in place mechanically by surrounding atoms.
Argon, an inert gas, is not chemically bonded to neighboring atoms at all, and even minor thermal disturbance allows them to escape.
If the rocks have an interbedded lava flow or volcanic ash bed, it's gold.
The older our sample is, the more daughter isotope it will contain relative to the parent.
Furthermore, Parentium and Daughterium are so different in chemical properties that they don't otherwise occur together.
If there were such a pair of isotopes, radiometric dating would be very simple.
Sedimentary rocks are generally hard to date because common cements like silica don't have datable radioisotopes, and minerals like glauconite that are common in sedimentary rocks are very prone to resetting.
If only there were long-lived isotopes of silicon, calcium, and magnesium!
So accurate determinations require very pure samples, very accurate and selective detectors, or both.
The true age of a sample is self-explanatory, but unless the material dates from historic times, the true age is rarely known.
Uranium-lead dating methods often use this approach because some of the minerals used in dating lose the lead decay products over time.