BBC Knowledge Asia Edition - December 2014

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There are 7 billion humans and

around 200 billion birds in the world.

That seems like a lot of two-legged

animals, but non-human mammals

number around 500 billion and

almost all of them are four-legged.

And according to some studies,

there might be another 10 trillion

reptiles and amphibians. So the

mean number of legs for land

vertebrates is very close to 4 (it’s

actually about 3.96). Fish don’t

have any legs of course, and there

are another ten trillion of those, so

including them brings the average

back down to 2.

But this is only for vertebrates.

What if we included invertebrates

as well? We can ignore starfish,

octopuses, spiders and centipedes:

they have lots of legs each, but

there just aren’t enough of them to

affect the average. But there are an

estimated 5 million trillion (5x 10^18 )

insects. And in the sea there are at

least 20 times as many copepods –

zooplankton with between 6 and 10

legs each.

In the end though it makes no

difference, because all of them

are outnumbered by the humble

roundworm. Nematodes are

mostly microscopic, but they’re

multicellular and so count as

animals, and they are found in high

densities in virtually every Earth

ecosystem. The 10 billion trillion

nematodes in the world drag the

mean number of legs down to less

than 0.1, which means that on

average, animals don’t have any

legs at all! LV

What is the average number

of legs for an animal?

Four legs good, 0.1 legs better

• at least where statistical
  averages are concerned

How is a fracking drill

turned through an

angle of 90 degrees?

A fracking drill bores its way through

a vertical well within shale rock. Then it

turns horizontally, injecting high-pressure

water and chemicals, forcing out reserves

of gas. The drill bit is made up of sharp,

rotating teeth. The head is slightly angled

at about two degrees and can be steered.

Accelerometers and other sensors relay the

bit’s position, helping the operators keep it

on track. GM

The head of a fracking

drill is tilted and can be

steered remotely

Just as the flow of water is driven by

differences in height, the flow of

electricity is affected by the ‘potential

difference’ between two points, measured

as voltage. The earth represents zero

potential, so all electricity will flow towards

it if given the opportunity – say via a wire or

lightning strike. RM

Why does electricity

always run to ‘earth’?

Lightning occurs when
clouds and the Earth have a
large potential difference
PHOTO: ALAMY, GETTY X3, THINKSTOCK, NASA