Mostly air, in tiny boxes
Why Does Cork Float?
Cork is more than half air, sealed into millions of separate little rooms that water cannot get into.
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Step 1 of 5
It is bark, and the tree survives.
Cork is the thick outer bark of one kind of oak. It can be stripped off carefully every nine years or so, and the tree simply grows it back — which makes it unusually kind harvesting.
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Step 2 of 5
Every cell is a sealed box of air.
Under a microscope cork is a honeycomb of tiny closed cells, each with air trapped inside. More than half of a cork is that air, which is why a lump of it weighs almost nothing.
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Step 3 of 5
Sealed cells will not let water in.
Wood soaks up water because its tubes are open. Cork's cells are closed and coated in a waxy substance, so water cannot get in — it floats, and it stays floating.
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Step 4 of 5
It squashes without spreading.
Squeeze most things and they bulge sideways. Squeeze cork and the air in the cells just compresses, so it gets narrower — which is exactly what lets a cork be pushed into a bottle neck.
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Step 5 of 5
The same air blocks heat and sound.
Still air is a poor carrier of heat and sound, and cork is full of it. That is why it turns up as floor tiles, wall panels and the lining of things that need to stay cool.
A cork's grip in a bottle neck can last decades, but it slowly dries out — which is why wine bottles get stored on their side.
The short version
Cork is bark full of sealed air pockets, so it floats, squashes narrower, and blocks heat and sound.
Try it yourself
Push a cork under water and let go. Then try the same with a piece of wood the same size, and wait.
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