Hmm, what are they doing?

Mum & Dad took me for a walk along the canal today. It was nice and warm in the sun and quite far from the noisy big lorries; I don't like them very much. Then I saw these two humans with what looked like string on a stick. They were dipping the string in the canal water. No idea why but they seemed quite happy just standing there.
That was my third walk today and I am wondering if there will be another one later. My little legs are tired!