Once upon a time there was a pond...

Once upon a time there was a pond...a small, barren, neglected pond. Then weed blew in and covered the surface of the water until there was nothing to be seen bar a green carpet.
Ducks and ducklings arrived.
Within a week, the weed had been eaten, the clear water dulled and polluted and the barren look returned. Yet the pond was not neglected. Oh no, Mama and her faithful four visited it several times a day; swam on it; dived into it; bathed in it. It had LOTS of attention.
The ducklings grew up. Mama left. The young grew wings, fledged and flew away.
The mucky water cleared and then, one day, the pond weed returned.

And multiplied.
And exploded!