Michael's teeter-totter log. He had started walking across the log from the left side when I told him his route was a dead end. Undeterred he continued towards the end of the log. He hadn't gone far when his weight (only 75 lbs) was enough to make the right end of the log drop down to the ground on the other side of the stream. Without missing a step he continued across the log triumphantly stepping to the ground after which the log rotated back to its original position.