Autumn has arrived, and at Granite City Elementary School everyone is gearing up for the biggest and best event of the year, the Harvest Festival. The whole school is excited about the games, the contests, the food, and most of all--the costumes Everyone except Lucy. She doesn't like dressing up, and has no desire to be a fairy princess or rock star, even for one day. pBut Lucy is excited about the new science unit Miss Flippo has started the states of matter. Lucy and her friends understand solids and liquids. They're easy. But gasses are more difficult to grasp. pWhen the class goes on a field trip to an orchard and Stewart Swinefest eats too many apples, and gets a serious stomachache, Lucy suddenly understands that even if you can't see gasses they can fill space and expand, and even make you move. pAnd with Stewart feeling better, she has a really great idea for her Harvest Festival costume, too. pThe second book in a new chapter book series from IRA Children's Book Award-winner,