[This is an ABC podcast]

Good day, Dr Karl here - Sooner or later, most young children will ask the question, "Why is the sky blue?". And another big one for curious, developing minds is: "Do fish drink water?".

To save you from blundering through an answer alone, I've done the research and prepared one for you.

The solution to this conundrum is that some fish do drink water, and some don't. Put simply, in general, saltwater fish drink lots, while freshwater fish don't.

Plus, surprisingly (and perhaps entertainingly for the young child asking the question), despite saltwater fish drinking lots of water, they only wee out only a small amount of urine — while freshwater fish do the opposite.

To explain further, there are two things that we need to understand first. Act 1: "getting oxygen to live", and Act 2: "osmosis".

Let's start with Act 1: oxygen.

We humans have it easy. We are surrounded by oxygen — it makes up one fifth of the air we breathe. So we simply have to breathe in, and inside our lungs oxygen diffuses across some 70 square metres of a very thin membrane into the blood. Bingo! Problem solved, we've got some oxygen into our blood.

But fish have it harder. In their environment, oxygen is dissolved in the water all around them — but it's trickier to get to. They can't break down the water molecule (which is H2O — two atoms of hydrogen and one atom of oxygen). No, they have to rely entirely on oxygen molecules that might happen to be there, lurking in between the water molecules.

Fish have been around for about 450 million years, and they've evolved gills to remove the oxygen from the water. Gills are light feathery structures with lots of surface area on the outside, and blood vessels on the inside. At the surface of the gill, there's a barrier between the flesh of the fish and the outside water and it's called a "semi-permeable membrane". Some things can cross the barrier, others can't. Oxygen molecules are small enough to passively diffuse - or pass - from the water into the blood of the fish. But any salts in the water (sodium, chloride and so on) are too big. Water molecules are small enough to passively diffuse across — but this action depends on which is saltier: the fish or the water around it.

And this brings us to Act 2: the strange concept of "osmosis".

To demonstrate, let's suppose you have a fish tank, half-filled up with freshwater. Then, you put a "semi-permeable membrane" all the way across the tank, at the half-way point along the length of the tank. And remember: a membrane like this functions like a fish's gills. Now, initially, the water level on each side of this membrane remains the same — as you would expect. But, then you dissolve some common table salt into the water on one side of the membrane. Suddenly, the water level on this side begins to rise. What's going on? Fresh water on the other side is diffusing across the semi-permeable membrane, to try to balance the level of saltiness on either side.

This is a natural balancing act, that just happens for various complex reasons that we won't go into.

So, let's now look at how this applies to a fish, starting with a freshwater species.

In a non-salty pond or river, the inside of the fish is saltier than the water around it.

So, just like in the tank experiment earlier, the water molecules in the river or pond will flood into the body of the fish — via the semi-permeable membrane of the gills and the skin. So, the freshwater fish will be flooded with water. This means a freshie has no need to actually drink water — because it's already getting a deluge via the gills and skin. In fact, the freshwater fish is at risk of swelling like a balloon, from all this water. So to get rid of this excess water, it has big kidneys to help get rid of lots and lots of urine. Best not to think about that next time you swim in a freshwater pond full of fish.

So, freshies don't drink much, but they naturally swell with water passing into their bodies, and then they have to wee it all out or risk popping.

The opposite happens with a saltwater fish in the ocean. The inside of a fish here is less salty than the ocean. So the water inside the fish floods out, in a futile attempt to dilute the entire ocean. Their problem is shrivelling up from a lack of water. So saltwater fish drink water like crazy. And they definitely don't need to make lots of urine, so they have only small kidneys.

So, when it comes to the question of do fish drink, all you have to remember is that salties swallow the sea, while freshies fatten and wee.