When visiting Chambord castle in France, dad suggests Christopher going up to roofs by the central spiral staircase.
He asks him to begin the ascent while he takes some photos.
Along the staircase, openings, allowing inward views, allow Christopher to see dad on the other side, at the same height. The ascent continues : dad and Christopher take the same staircase, without ever crossing themselves ! When they arrive at the top, they finally join together at the roofs.
At Chambord, the main staircase is a double spiral one : two staircases, turning one in the other one, allow to climb without crossing each other.
