You take a sip of water.

It disappears.

And, unless it goes down the wrong way and starts a coughing fit, you probably don't give it another thought.

But where does it actually go?

Does it head straight for your cells? Hang around in your stomach? Get whisked off to your kidneys?

Let's follow it.

First stop: your stomach

Unlike food, water doesn't need to be broken down before your body can use it.

After you swallow, it travels down your oesophagus and into your stomach. From there, it begins moving into your small intestine, where most of the water you drink is absorbed.

And this can start surprisingly quickly.

In human studies using specially labelled water, traces have appeared in the blood within just a few minutes of drinking. How quickly the rest arrives depends on things like how much you drank, whether you've eaten and what else is in the drink.

Then it crosses into your bloodstream

The walls of your small intestine are rather good at moving water.

Water passes across the intestinal lining and into the circulation, joining the watery part of your blood.

Your bloodstream then becomes its transport network.

But here's where it gets interesting.

The glass of water you just drank doesn't remain a neat little parcel of "new water" being delivered around your body. Its molecules become part of the enormous, constantly moving pool of water that is already inside you.

Some stays within the blood.

Some moves into the fluid surrounding your cells.

And some crosses cell membranes to become part of the fluid inside your cells. Water shifts between these spaces in response to changes in the concentration of electrolytes and other dissolved substances.

Your body decides where it is needed

This is the bit I find particularly clever.

Your body doesn't simply absorb every glass and hope for the best.

It is constantly monitoring the concentration and volume of your body fluids.

If water is becoming scarce, your brain can trigger thirst and release vasopressin, sometimes called antidiuretic hormone, which tells your kidneys to conserve more water.

If there is more water than your body needs, your kidneys can allow more of it to leave in your urine.

Healthy kidneys continually filter the blood, returning almost all the water your body wants to keep while sending excess water and waste towards the bladder.

So your kidneys aren't simply an exit door for the water you drink.

They're part of the control room.

And then?

Some of that water may become part of your blood plasma.

Some will sit between your cells.

Some will enter the cells themselves.

Some will help regulate temperature and eventually leave as sweat.

Some will return to the digestive system.

Some will leave through your breath.

And some will eventually become urine.

The molecules are continually moving, crossing boundaries and being recycled through different parts of you.

Which makes drinking a glass of water feel rather less ordinary.

The curious bit...

Hydration, then, isn't really a story about filling an empty tank.

We're not bottles.

We're dynamic systems in which water is continually arriving, circulating, moving between compartments and leaving again.

That glass on your desk?

Within minutes, some of its water may already have joined the river running through you.

And that raises another question...

If water can enter the bloodstream within minutes, does drinking more at once mean you become hydrated more quickly?

That feels like a question for another Curious Hydrologist.