When we die, we no longer ingest C14, and it begins to decay and turn into N14.
By comparing the amount of C14 in an object to the amount of N14 in it we can determine how long it has been decaying for, and therefore when the organism died. Through decay Uranium-238 turns into stable Lead-206.
As long as you follow these four steps you will always be able to accurately determine the age of a rock or fossil.
Many rocks and organisms contain radioactive isotopes, such as U-235 and C-14.
These radioactive isotopes are unstable, decaying over time at a predictable rate.
The layers of sedimentary rock, or strata, can be seen as horizontal bands of differently colored or differently structured materials exposed in this cliff.
The deeper layers are older than the layers found at the top, which aids in determining the relative age of fossils found within the strata. Such index fossils must be distinctive, globally distributed, and occupy a short time range to be useful.