One of the most impressive hot pools in Yellowstone is called the Morning Glory pool. It is famous for its deep colors and its otherworldly looks. But over the years, the pool has lost much of its color, according to reports--I wasn't able to visit Morning Glory Pool on Road Trip 2009--because of a wide variety of detritus thrown into it by park visitors.
This sign posted next to another beautiful hot pool is a warning--and an explanation--to visitors that they must not think of the pools the way they think of fountains. The junk gets stuck deep in the pools' pipes, clogging them and making it impossible for the pools to function as nature intended them to.