Do fish drink water? Are there any animals that don't need to drink any?

A kangaroo rat doesn't have to drink water.

The answer to your question depends, of course, a bit on what you mean by "drink." Freshwater fish don't actively drink water, but they do of course absorb water through their skin and gills. Saltwater fish actually take in water through their mouths - which is something closer to what you'd think of as drinking - and get rid of salt with the help of their gills.

Fish like salmon, which live part of their lives in salt water and part in fresh water, can do both, with the choice being determined by where they are. Dolphins (which are mammals, not fish) get their water from the fish they eat.

All animals we know of need water one way or another. Almost anything you'd think of as food contains water, but most animals need to drink some liquid in addition to what they eat.

Probably the most remarkable creature in terms of doing without anything like drinking is the kangaroo rat. They have incredibly efficient kidneys and are very good at holding on to water. The amazing thing is that they go one better and actually make their own water. If you burn carbohydrates, you get carbon dioxide and water as waste products, and if you do this "burning" in a living thing you can actually (if you're a kangaroo rat) recover some of this water. As long as they have dry food to eat they never have to drink.

Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@ globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.