Question: What are the little black gnats that fly out of my houseplants when I move them or water them?

Answer: Those are fungal gnats. They're more annoying than they are damaging. They are indicators of keeping your plants too wet. They develop in the algal scum that grows on top of wet soil, also on the walls of pots. Control them by (1) watering slightly less often, (2) replacing the top half-inch of potting soil with fresh soil and (3) drenching the soil with a houseplant insecticide.