Sentence:  “I'm a wheel man,” he says. “I've been crazy about Ferris wheels since I was a kid. The Ferris wheel is the <WSD>key to the midway</WSD>. When it's up and running and the lights are on, everybody knows the show is open.” 
Meaning of the word marked with <WSD> tags:  A Ferris wheel or carousel, due to their central location.