On February 26, you might see a few people going about their normal business with black, cross-shaped smudges on their foreheads. That’s because it’s Ash Wednesday, when Christian churches across the world mark the beginning of Lent by anointing each member of the congregation with an ashen cross. Considering that millions of Christians participate in the tradition, those ashes definitely add up.

So where do churches get them? As Reader’s Digest reports, the answer is related to another Christian observance: Palm Sunday. Each year, churches hold a special Mass on the Sunday before Easter to commemorate the day Jesus arrived in Jerusalem before his crucifixion. In the Bible, a crowd of worshippers welcomed him by waving palm fronds as he rode into the city on a donkey—so, on Palm Sunday, churches hand out palm fronds to their members. (The palms, according to TIME, are purchased from suppliers based in locations with tropical or subtropical climates.)

While some people twist their dried palms into the shape of a cross and take them home, most are left in the church and later burned. The ashes from that fire are stored and used on Ash Wednesday the following year.

Unlike the tradition of passing out palm fronds, the practice of blessing churchgoers in ash isn’t directly linked to the Bible. Instead, it’s a symbolic way to remind Christians of their own mortality. When a member of the clergy marks your head in ash, they’ll often say, “Remember that you are dust, and to dust you shall return,” which is what God told Adam when he banished him from the Garden of Eden.

For more on the meaning and history of Ash Wednesday, check out these facts.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.