only 10% of our brain.

The other 90% standing idly by for spare capacity.

Hucksters promised to unlock that hidden potential

with methods "based on neuroscience,"

but all they really unlock is your wallet.

Two thirds of the public

and nearly half of science teachers

mistakenly believe the 10% myth.

In the 1890s, William James,

the father of American psychology,

said, "Most of us do not meet

our mental potential."

James meant this as a challenge,

not an indictment of scant brain usage.

But the misunderstanding stuck.

Also, scientists couldn't figure out

for a long time

the purpose of our massive frontal lobes

or broad areas of the parietal lobe.

Damage didn't cause motor or sensory deficits,

so authorities concluded they didn't do anything.

For decades, these parts

were called silent areas,

their function elusive.

We've since learned that they underscore

executive and integrative ability,

without which, we would hardly be human.

They are crucial to abstract reasoning,

planning,

weighing decisions

and flexibly adapting to circumstances.

The idea that 9/10 of your brain

sits idly by in your skull

looks silly when we calculate how the brain uses energy.

Rodent and canine brains consume

5% of total body energy.

Monkey brains use 10%.

An adult human brain,

which accounts for only 2% of the body's mass,

consumes 20% of daily glucose burned.

In children, that figure is 50%,

and in infants, 60%.

This is far more than expected

for their relative brain sizes,

which scale in proportion to body size.

Human ones weigh 1.5 kilograms,

elephant brains 5 kg,

and whale brains 9 kg,

yet on a per weight basis,

humans pack in more neurons

than any other species.

This dense packing is what makes us so smart.

There is a trade-off between body size

and the number of neurons of primate,

including us, can sustain.

A 25 kg ape has to eat 8 hours a day

to uphold a brain with 53 billion neurons.

The invention of cooking,

one and half million years ago,

gave us a huge advantage.

Cooked food is rendered soft and predigested

outside of the body.

Our guts more easily absorb its energy.

Cooking frees up time

and provides more energy

than if we ate food stuffs raw

and so we can sustain brains

with 86 billion densely packed neurons.

40% more than the ape.

Here's how it works:

Half the calories a brain burns

go towards simply keeping the structure intact

by pumping sodium and potassium ions

across membranes to maintain an electrical charge.

To do this, the brain has to be an energy hog.

It consumes an astounding 3.4 x 10^21 ATP molecules per minute,

ATP being the coal of the body's furnace.

The high cost of maintaining resting potentials

in all 86 billion neurons

means that little energy is left

to propel signals down axons and across synapses,

the nerve discharges that actually get things done.

Even if only a tiny percentage of neurons

fired in a given region at any one time,

the energy burden of generating spikes

over the entire brain

would be unsustainable.

Here's where energy efficiency comes in.

Letting just a small proportion of cells

signal at any one time,

known as sparse coding,

uses the least energy,

but carries the most information.

Because the small number of signals

have thousands of possible paths

by which to distribute themselves.

A drawback of sparse coding

within a huge number of neurons

is its cost.

Worse, if a big proportion of cells never fire,

then they are superfluous

and evolution should have jettisoned them long ago.

The solution is to find

the optimum proportion of cells

that the brain can have active at once.

For maximum efficiency,

between 1% and 16% of cells

should be active at any given moment.

This is the energy limit

we have to live with

in order to be conscious at all.

The need to conserve resources

is the reason most of the brain's operations

must happen outside of conciousness.

It's why multitasking is a fool's errand.

We simply lack the energy to do two things at once

let alone three or five.

When we try, we do each task less well

than if we had given it our full attention.

The numbers are against us.

Your brain is already smart and powerful.

So powerful, that it needs a lot of power

to stay powerful.

And so smart

that it has built in an energy efficiency plan.

So don't let a fradulent myth

make you guilty about your

supposedly lazy brain.

Guilt would be a waste of energy.

After all this,

don't you realize it's dumb to waste

mental energy?

You have billions of

power-hungry neurons to maintain.

So hop to it!