Oscar pinched the clamp onto his left ear. The device emitted a little wordless burbling hum, the sound a contented three-year-old might make. As long as he moved with the crowd, the little murmur simply sat there at his ear, an oddly reassuring presence, like a child’s make-believe friend. However, if he interfered with the crowd flow—if he somehow failed to take a cue—the earcuff grew querulous. Stand in the way long enough, and it would bawl.