A visual analogy depicts the challenges of aquatic animals in fresh water and salt water. For fresh water, three stages are described using art and the following text:

• First stage: The bodies of freshwater animals, such as fishes, contain a higher concentration of salt than the water they live in.
• Second stage: So water moves into their bodies by asmosis, mostly across the gills. Salt diffuses to. If they didn't excrete water, they'd look like water balloons with eyes!
• Third stage: So they excrete water through kidneys that produce lots of watery urine. They don't drink, and they actively pump salt in across their gills.

For salt water, three stages are described using art and the following text:

• First stage: The bodies of saltwater animals, such as fishes, contain a lower concentration of salt than the water they live in.
• Second stage: So they lose water through asmosis, and salt diffuses in. If they didn't conserve water and eliminate salt, they'd shrivel up like dead leaves.
• Third stage: So they conserve water by producing very little concentrated urine. They drink, and they actively pump salt out across their gills.