Why do people put coins on headstones?

People leave coins on headstones to show that they visited the grave, according to Snopes.com. The practice may stem from Greek traditions, in which the deceased were buried with a coin in their mouth in order to pay a mythical ferryman to take them to the land of the dead.

It is common in some cultures and belief systems, including Judaism, to leave a coin or a pebble on a gravestone in order to pay respects and show that people were visiting the grave. This practice became popularized as a U.S. military tradition, where the size of the coin indicated how the visitor knew the deceased.