To see that water molecules move around even when the water seems to be very still, take a full glass of water, cover it with a piece of plastic wrap, and poke a small hole in the plastic wrap. Wait for two days without disturbing the glass, until you are sure the water is not moving. Carefully drop one drop of red food coloring into the water through the hole in the plastic wrap, disturbing the water as little as possible.
At first, the water is only red in one place. But gradually over several days the red will spread through the water until it is evenly distributed. That’s because the water molecules are moving.
What do you think would happen if you tried the same thing with very hot water? How about very cold water?
Most fish and reptiles can’t control their own body temperature. They just are the same temperature as the air or water around them. How do you think it would affect them if the water or air around them got hotter or colder?