Elizabeth & I were cycling home from the grocery store the other day, and were behind this group of boys - probably aged 8-11-ish. We went over the Cam, and halfway across the bridge, one of the boys threw some garbage into the waters. We eventually caught up to them, and I cleverly said to this kid, "Hey, can you not litter? I saw you throw that garbage into the river." End.
This kid just looked at me and said, "yeah, sorry", and then I biked home.