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David Attenborough is in the high moorlands of Peru, 15,000 feet above sea-level. Standing next to a tiny stream he explains that water is the only substance on earth that stays a liquid at normal temperatures and pressures. Ninety-six per cent of the water on earth is salty but streams like the ones next to him contain water that has been distilled from the surface of the sea by the heat of the sun. It then rose into the sky as vapour, condensed as clouds and fell again as rain and snow to make streams of pure fresh water. But the stream next to David is special in that is one of the many that can claim to be the source of the Amazon, the largest river on earth. Torrent ducks are strong swimmers here which is just as well because the Amazon water they live in runs down the hill at great speed. The ducks exploit the swirls and eddies with consummate skill, paddling with powerful strokes of their large webbed feet. Always facing upstream, they brace themselves against the rocks using their stiff quilled tails and use the small horny spurs on the wrists of their wings to give them purchase. Pairs own their own stretch of the river. They work their way up the river until they reach the frontier of their territory and then let themselves be swept downstream to begin all over again.