Why a puffy cloud can have such a straight bottom
Follow warm air upward to see how condensation creates a roughly level cloud base—and how drier air helps shape the fluffy edges above it.
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How corals build thriving reefs in nutrient-poor water
Coral animals share their tissues with sunlight-powered partners. Follow the exchange to see how it supports reef growth—and what bleaching interrupts.
Free reads
There is no final prime.
An ancient argument defeats every list you could ever finish.
Five words with other passports.
Ordinary English carries traces of journeys you may never have noticed.
One message. Three ways to write it.
The Rosetta Stone mattered because part of its mystery came with a readable comparison.
A tiny tunnel closes your jacket.
The zipper's useful part is hiding inside the bit you pull.
A forest with its feet in the sea.
Mangrove roots are a shoreline, a breathing system and a neighbourhood.
The sky's almost-still point.
Polaris is useful because of where it is, rather than how brightly it shines.