Final answer:
The Greeks believed that a coin had to be placed under a dead person's tongue so their soul could pay the ferryman Charon to cross the river Acheron to the underworld.
Step-by-step explanation:
The Greeks believed that when a person died, their soul had to pay a fee to the ferryman Charon in order to cross the river to the underworld. To ensure that the deceased's soul could pay this fee, the Greeks would place a coin under the person's tongue. The river that the dead had to cross was called the Acheron. This custom of burying a deceased with a coin for the ferryman Charon became visible in Athens in the late fifth century BC.