Our first indication that something was up was when BILJ ran into the house and back out. He'd heard the neighbor's smoke-detector alarm sounding, saw a living room filled with smoke, and went for a fire extinguisher and the neighbor's house key. He couldn't find the key, but soon after he was back over there, the firepersons showed up. I"d gone over to be helpful, but once the Grown Ups In Charge arrived, I went and hung out with Hz to stay out of the way.
It turned out to be a smoldering sofa* caused by the sun shining through... A SNOW GLOBE. Yes, a "harmless" snow globe on the windowsill focused the sunlight onto the sofa material and got it going. If BILJ hadn't heard the alarm when he did, it might have ignited, and the damages would have been far worse than a badly charred sofa and a lingering smoke smell.
So be safe, Everyone! Check your smoke alarms and fire extinguishers, and make sure your snow globes aren't trying to burn your house down!
* Note: "Smoldering Sofa" would be a great name for a progressive Ska accordion band.