Almost entirely water, on purpose
Why Is a Cucumber So Watery?
A cucumber is about ninety-six parts water in a hundred, and that is the whole design.
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Step 1 of 5
Plants hold themselves up with water pressure.
A cucumber stands up the same way an inflatable does — by pressure, not by strength. A plant cell is a bag pumped tight with water, pressing against a stiff wall. Millions of them pressed tight is what makes a stem stand and a cucumber crunch.
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Step 2 of 5
Which is why it goes limp so fast.
Let the water out and the cells go slack. Salt a cucumber, or leave it a week, and it loses pressure — nothing is broken, it has just deflated.
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Step 3 of 5
The water is why it feels cold.
Water takes a lot of energy to warm up, so a cucumber holds its coolness. It really is cooler than the air around it for longer than most food.
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Step 4 of 5
Salting is a way to use it.
Sprinkling salt draws the water out on purpose, which concentrates the flavour and stops a salad or a sandwich going soggy an hour later.
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Step 5 of 5
Pickling replaces the water with something hostile.
Vinegar or salty brine soaks in where the water was. Nothing that would spoil the cucumber can live in it, so a pickle keeps for months.
Cucumbers are botanically a fruit — a berry, in fact — because they grow from a flower and carry the seeds.
The short version
A cucumber is water held under pressure in a cell scaffold, which is why it is crisp, cool, and quick to go limp.
Try it yourself
Salt a few cucumber slices and come back in twenty minutes. The puddle underneath came from inside them.