Glenwood Puns

So we were in glenwood springs and went into the caves. It seemed like a really normal tour and just as we're about to exit, the guide tells us to put our ears on the wall of the cave. We all did assuming it was some cool feature of the cave. The guide asks us what we think we're listening to. Some reply water and others reply erosion. The guide tells us we're wrong and all we're doing is listening to hard rock.