One line of evidence involves rocks from outside the Earth--meteorites and moon rocks.
Many elements have naturally occurring isotopes, varieties of the element that have different numbers of neutrons in the nucleus.
Radiometric dating works best on igneous rocks, which are formed from the cooling of molten rock, or magma.
As magma cools, radioactive parent isotopes are separated from previously formed daughter isotopes by the crystallization process.
Over time, radioactive isotopes change into stable isotopes by a process known as radioactive decay.
Some radioactive parent isotopes decay almost instantaneously into their stable daughter isotopes; others take billions of years.
The rates of decay of various radioactive isotopes have been accurately measured in the laboratory and have been shown to be constant, even in extreme temperatures and pressures.