Bioremediation
Bioremediation is using living things like bacteria or enzymes to clean up pollution.
Think of it this way
A jar of greasy water sits on the counter. You drop in tiny beads that soak up the grease and break it down. The water turns clear as the beads do their work. The jar holds the same water, but the dirty stuff is gone.
Explaining bioremediation by grade level
Some tiny living things can eat grease and oil. You can not see them, but they do big work. They munch through the grease and break it into safe bits. The dirty water gets clean over time. Nature has its own clean-up crew.