If you grew up celebrating Halloween, you likely have fond memories of rushing from house to house collecting candy. But that tradition has an interesting history. While activities reminiscent of today's trick-or-treating have been around since ancient times, actual candy hasn't been part of it for as long as you think.

The holiday of Halloween can trace its origins back more than 2,000 years, to the ancient Celts and their harvest celebration, Samhain. This celebration is also a big reason the traditional colors of Halloween are black and orange. Celebrants believed that halfway between the autumnal equinox and the winter solstice, around October 31 and November 1, the gateway between the worlds of the living and the dead was weakened. So they would prepare for the presence of deceased spirits by offering up food and drink to the wandering spirits.

As the holiday evolved into the Middle Ages, it gained religious significance. People now participated in All Souls' Day, on November 2, by going to wealthier people's homes and offering to pray for their deceased loved ones in exchange for money or food. And this practice, known as "souling," evolved into a fun one for children, who would also go from home to home. But instead of offering prayers, they would sing or tell a joke and receive a treat, like coins or fruit, in exchange. Starting to sound familiar? Here are 12 things you never knew about Halloween candy.

Before the 19th century, these trick-or-treating precursors were really only popular in Ireland and Scotland. But during that century, immigrants began coming to the United States from these countries, in many cases to avoid famine. They brought the celebration, now known as Halloween, with them.

Now in semi-modern times: the holiday gained popularity, especially among young people, in the early 20th century before celebrations declined due to the Great Depression and World War II. And when people did "celebrate," it was with more malicious pranks like vandalism. But once the 1950s hit, the holiday came back—and the increasing suburbanization helped establish the practice of kid-friendly trick-or-treating as we know it today.

As for the candy? Well, candy companies in the post–war ration era were growing, and they began realizing what a gold mine this holiday could be. That's how we got the now-ubiquitous Halloween-themed candy surge that descends upon our retailers every fall. Today, Americans spend a lot of money on treats for this holiday. By the way, here's how long Halloween candy really lasts.