Wording of this Handout:
Imagine you were a fish with gills that needed cleaning. You could not reach round with your mouth because fish cannot bend that far, and you could not poke something into your gill in case you damaged it.
You could not get another fish to help because (a) it might eat you, and (b) fish don't have mouths which can reach into tight corners like gills.
God has solved the problem. He created some shrimps which enjoy cleaning fish! The shrimp sits on the sea floor waving its feelers (antennae) about. ("Yoo hoo, I'm waiting for a client!")
Along comes the fish and the shrimp gets to work. It goes all over the fish, cleaning the scales, biting any nasties, and grooming every part, then the fish swims away. ("Thanks mate!)
The fish needs the shrimp, and the shrimp needs the fish.
God is a great designer. He has made many things which work together like this.