Input (grade 13): Since water does not usually go up to the surface of the sand, the sand on top appears solid and can support leaves and other small debris, making quicksand difficult to distinguish from the surrounding environment.
Output (grade 9): Since the water does not usually go all the way up through the sand, the sand above does not appear to move at all, and can support leaves and other small debris. This makes it difficul to distinguish quicksand from the surrounding environment.