Oldest tricks new dog

Flossie had been playing with her friend Bud, an ancient Grandpa type lurcher greyhound cross. She could hardly sit her backside still until Dad held up the magic piece of cheese above the camera.
She has no fear and went swimming determinedly to capture a piece of polystyrene she has passed countless times. Result, look away now if you don't want to know the score.

Flossie 1 - Polystyrene pieces. There will not be a return leg. She nearly picked up and swallowed a piece of glass. Why do humans do it? Why leave a bottle or shard in a beautiful dog friendly location?