Under the microscope a drop of pond scum can give hours of delightful study. What feels between the fingers like slime is, microscopically, an open lake crowded with queer vegetable and animal life.

I have a garden pond that suffers from “pea-soup” algae. A thunderstorm recently flooded the nearby road, overflowing into my garden and the pond. The next morning the algae had gone. What was in the ...