It’s October 31, which means just one thing: Halloween. But when and how did Halloween begin? And why do millions of children dress up and go out Trick or Treating? National Geographic has the answers.

This relatively short video spells out the history of Halloween and some of the traditions associated with it.

The celebration which became Halloween has been around for 2,000 years, with the Celts of Europe believing that Oct. 31 was the day when the worlds of the living and the dead came together. They lit bonfires to keep spirits away.

Religion then got involved, as it so often does, and Nov. 1 was labeled ‘Hallowmas’, or ‘All Saints Day’. Oct. 31 was therefore All Hallows Eve, which gradually morphed into Halloween.

Trick or Treat began as a form of extortion by poor Irish immigrants in the U.S. Giving treats to the youngsters became a way or avoiding vandalism, and so grew what has now become the main Halloween tradition.