I was in my hometown of Boulder, Colorado over the weekend for a family reunion. On Friday afternoon we had a picnic at Eben G. Fine Park, which is bordered on one side by Boulder Creek. I noticed some stone sculptures coming out of the water and walked over to snap a photo (above, click to embiggen). I thought they were a permanent fixture, and that the rocks had been cemented together, because they looked impossibly balanced.
But a couple of days later I was looking in the Boulder Daily Camera, and I discovered that the rock sculptures were not cemented together. They had just been placed there earlier in the day by a man named Mike Grab. He has a website with photos of his sculptures, called Gravity Glue.