Tom and his family were vacationing on a car trip across the country. Every so often his father (or, more often, his mother) would grab a pamphlet at a diner and exclaim, "Oh, wow!" and convince the rest of the family that the Barbie museum (the doll, not the Klaus) would be a good spot to spend an
hour or so. Or the Edison home. Or the world's largest milk-box-shaped restaurant.
His father had been roundly booed after the World's Third Largest Ball of Twine and hadn't really spoken up, except to meekly order lunch, in the last few days.
But no matter, for the purpose of the trip, the REAL purpose, was now within sight. Family together time be damned, because Father had just parked the car outside of the Hourglass Bridge.
A large, functional hourglass timer sat in the center and could easily be seen from either side of the crossing. Rather than be filled with sand, it contained large shiny metal spheres, that would slide through the center and land upon their fellows below with a serene "CLONK!"
When the last sphere fell through, a time delay ensured that the device was, in fact, empty. Then, safety rails rose up at the ends of the bridge. Then, another delay for safety - and finally, the magic!
Two locks engaged what had up those point been freely-rotating waterwheels below the bridge, and others released the bridge to rotate. Powered by the river, the center portion of the bridge, itself in the shape of an hourglass (but cleverly weighted to ease the work on the cams within, Tom would later learn) slowly rotated 180 degrees.
Those who remained on the bridge, like Tom and his family, went from east to west in a gentle rotation over the course of just a few minutes.
The same gears that rotated the bridge were linked to the hourglass itself, and flipped it over during the bridge's journey, to begin the process anew.
As the bridge slowly slid into position, the locks disengaged the wheels and re-engaged the bridge itself, and the safety rails slowly dropped back down into the bridge decking.
It would be a disaster for rushed commuters who got spun about, of course, but the hourglass footbridge itself was by far the coolest thing Tom had seen on this family bonding experience - except, perhaps, the llama farm in West Virginia.
*inspired by a misreading of "Hourglass Badge," by [bumhat] (link).