BBC Knowledge Asia Edition - December 2014

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How do hawks

hover in the sky?

Birds of prey use a technique called

‘wind hovering’. This is really just flying

into a headwind, but they control their

forward airspeed so that it exactly

balances the speed of the wind and the

bird makes no progress relative to the

ground. Kestrels are the masters of this

What happened

to the missing

Stonehenge stones?

We don’t know for sure that they were

ever there. The incomplete outer ring of

stones today certainly suggests a circle, but

the archaeological evidence is inconclusive.

It’s entirely possible that the builders always

intended for it to be a crescent, or simply

ran out of stone or motivation. Some stones

have been removed, though. An altar stone

described by architect Inigo Jones in 1620

vanished soon afterwards. In 2007 it was

found in two pieces in a nearby village. LV

Combustion is simply a type of chemical

reaction that occurs between a source of

fuel and a source of oxygen, creating heat

plus new compounds. A source of energy

is often needed to split apart the fuel and

oxidiser molecules – for example, a spark.

But once the fragments start reacting, the

heat produced keeps the process going. RM

and can even hover briefly in stationary

air by balancing forward-gliding flight

with backwards-angled wingbeats. Gaps

between flight feathers at their ends

allow the bird to control wing turbulence

and avoid stalling, and fanning the tail

wide provides extra lift. LV

PHOTO: JONATHAN MITCHELL/ATLAS PHOTO ARCHIVE, THINKSTOCK X2, ALAN SIMKINS/HERITAGEACTION.ORG.UK

An altar stone that disappeared from Stonehenge in the

17th Century was rediscovered in a nearby village in 2007

Fire: a very

useful chemical

reaction indeed

The Galapagos

hawk; a master

of the art of

hovering flight

What makes things burn?