The mark received on Ash Wednesday, and why it stays on all day.
Ashes come from the burned palms of the previous Palm Sunday.
You will hear one of two formulas: remember that you are dust, and unto dust you shall return, or repent and believe in the Gospel.
Wear them out. Not showing them off, simply not scrubbing them off before returning to work.
Ashes say two things at once: you will die, and you can still turn. Lent only makes sense if both are true. The mark is also unavoidably public, which is how a private faith gets pulled into daylight once a year.
Sitting in ashes as a sign of repentance runs through the Old Testament, from Job to Nineveh to Daniel.
In the early medieval Church public penitents were marked with ashes at the start of Lent. When public penance faded, the whole congregation began receiving them, which is the quiet claim that everyone in the building is a penitent.
Receiving ashes is not obligatory and Ash Wednesday is not a holy day of obligation. Fasting and abstinence that day are obligations for those bound by them, which is a different matter.