Pebble en(counters)

For a while as I have been lapping the Sports Centre I have encountered little piles of pebbles that change on a daily basis. I was intrigued by these for ages and then the mystery was finally solved. There is outdoor gym equipment at the side of the paths I plod along and people have been using the pebbles as counters to keep track of their repetitions. I saw a guy add a pebble as he came off the bars during his workout.