Harry pulled a wizard cracker with Fred and it didn't just bang, it went off with a blast like a cannon and engulfed them all in a cloud of blue smoke, while from the inside exploded a rear admiral's hat and several live, white mice. (PS12)
As in Wizard Crackers or Christmas Crackers: A tube of cardboard wrapped in fancy paper and twisted at both ends. Inside the tube is a strip of paper coated in gunpowder, which snaps (cracks) when two people pull the cracker apart. Inside the tube, there would be a paper party hat, a small gift and a very childish joke on a little slip of paper. Crackers are pulled at Christmas dinner or lunch on 25th December.
At Harry's first Christmas at Hogwarts he got lots of goodies from the Christmas crackers - a pack of nonexplodable luminous balloons, a Grow-Your-Own-Warts kit, and a wizard chess set.