A tornado, or twister, is a tube of rotating wind. About 1,000 twisters touch down in the United States each year. Find out more in Tornadoes, one of the titles in the Surviving series. Surviving is a series of AV2 media enhanced books. A unique book code printed on page 2 unlocks multimedia content. These books come alive with video, audio, weblinks, slideshows, activities, hands-on experiments, and much more.