Halloween has its roots in an ancient Celtic festival celebrating the new year and the transition of souls to the underworld. It was believed that on Halloween night, ghosts and other spirits could roam the earth. Now, children dress up and go trick-or-treating, asking for candy and playing small pranks. Pumpkins were once carved and used as lanterns to light the souls' way and protect against evil spirits. In medieval times, some criminals disguised themselves on Halloween to cause trouble, contributing to our modern tradition of wearing costumes. Today, some people also dress their pets in costumes for Halloween festivities.