Here's why:

* Absolute age refers to the actual numerical age of a rock, expressed in years. It is determined using radioactive dating methods, which measure the decay of radioactive isotopes within the rock.

* Relative age is a less precise determination of a rock's age, simply stating whether it is older or younger than another rock. This is often determined by the order in which the rocks were formed, using principles like superposition (younger rocks on top of older rocks).

So, when someone says a rock is "50 million years old", they are referring to its absolute age.