Exactly enough time

Dhruv sits by the window of a crowded bus in stalled traffic, his pocket atlas open on his knee, looking anxiously ahead. A school gate down the street is starting to close. — THE EXAM BUS · Dhruv left with exactly the usual time.

Dhruv left for the exam centre at exactly the time the journey usually takes.

Most days that is enough. Most days the road is ordinary.

Today the road was busier. There was no extra time to give.

He reached the gate two minutes after it closed. He stood outside for a while, watching it stay shut.

the same plan, two mornings

what an ordinary morning takes

The same plan, two mornings. The bar is the time Dhruv gave the journey. On most mornings the day ends inside it. On the morning of the exam it ran past the end.

The plan was the same both days. Only the road was not.

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The idea

Look at the gap in Dhruv’s plan. It leaves no room. It works only while the road stays ordinary.

People who design things for a living have a rule for this. It works for a road, a battery or a family’s money — the same room, three different shapes.

A LONGER LOOK

The idea

Design for the error you know is coming.

The error is not a mistake anyone made. It is the busier road, the delayed bus, the bill. You know one is coming. You do not know when.

A margin is room kept back on purpose, before it is needed. It is not what happens to be left over. It is set aside on purpose, for a day that has not come yet. It sits quietly until that day comes.

The difference is the mark. Somebody drew a line before the day began, and kept what lay on the far side of it. What is left over by accident has no line. Nobody planned it.

two things that look the same on an ordinary day

what the day used

whatever was left

Two things that look the same on an ordinary day. On the left, whatever happened to be left. On the right, room set aside before the day began, with a mark. A margin is the second one.

You do not know which day the road will be busier. Or which day the battery will run low faster. Or which month the money will be needed somewhere else.

You only know that some day, one of them will. The margin is for that day, whichever one it turns out to be. That not-knowing is exactly why the room has to already be there.

three days that are not ordinary. you do not know which one comes

an ordinary day

kept back

Three days that are not ordinary. Nobody knows which one comes, or when. The room is for whichever it is.

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What people believe instead

Dhruv’s father stands relaxed by the front door, telling Dhruv, backpack on and atlas in hand, not to leave early. — AT THE DOOR · “Exactly enough is enough. Why leave early?”

Dhruv told his Papa about the gate. “Twenty minutes is what the journey takes,” Papa said. “Leave when it takes twenty minutes. Why sit around wasting time?”

Dhruv did not argue. He thought about the road that day. Exactly enough had worked every ordinary morning. It had not worked on the one morning that was not.

That is the trouble with exactly enough. It works right up to the day it does not, and by then there is nothing left to add.

“And most mornings you would stand at the gate for nothing,” Papa added. Dhruv nodded. That was true too.

Papa was not wrong about most mornings. He was only wrong about this one.

as Papa sees it, and the morning that was not ordinary

twenty minutes, exactly enough

As Papa sees it, every morning fits inside twenty minutes. The morning that was not ordinary had nowhere to go.

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The same gap, twice more

A girl holds up her phone at the school gate, its screen dark and switched off, disappointed, as other children are met by their parents. — THE SCHOOL GATE → · Isha’s phone read ten per cent when she left.

Isha’s phone read ten per cent when she left school. On an ordinary day that gets her home.

Today her bus was delayed. She needed to call home. The phone went dark before the call connected.

Look at the shape. It is Dhruv’s gap again, in a different costume. A little more charge, kept back on purpose, would have been enough.

the same charge, two afternoons

getting home on an ordinary afternoon

The same afternoon, two charges. Ten per cent was never the problem. The afternoon it was not enough was. A little more, kept back, and the call goes through.

Naveen’s mother drops a coin into a small tin box on a kitchen shelf in the evening, while Naveen does homework at the table nearby. — THE KITCHEN SHELF · Naveen’s mother adds to the tin box, most months for nothing.

Naveen’s mother sets a little aside in a tin box every month. Most months the family’s one salary already covers everything, and the tin box sits there, seemingly for nothing.

One month a bill came that the salary alone did not cover. The tin box is what closed the gap.

It was never for the ordinary months. It was for this one. Nobody knew which month it would be. The habit did not need to know.

the same salary, two habits

what an ordinary month uses

The same salary, two habits. Most months the tin box is not needed. The month a bill comes, it is the only thing that is.

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Spot it

Two new plans. Ask the same question of both.

PUZZLE CORNER

A water tank is filled to exactly what the household uses on an ordinary day. What happens the day the tanker truck is late?

PUZZLE CORNER

A cricket team brings exactly eleven players to a match, no more. What happens if one of them falls sick that morning?

two plans. where does the day that is not ordinary land?

what the house uses on an ordinary day

Two plans, drawn the same way. Where does the day that is not ordinary land?

Look for the shape. Both plans work for an ordinary day only. Neither has kept anything back for the day that is not. Both look fine, right up until the day they are not.

the same two plans, on the day that is not ordinary

what the house uses on an ordinary day

The same two plans on the day that is not ordinary. Neither kept anything back. Both run past the end.

Dhruv sits cross-legged on his bed in the evening, tracing a route in his pocket atlas, thoughtful and pleased, his backpack hanging on the wall. — THAT EVENING · Dhruv checked the route before he needed to.

Before you trust a plan, ask one question. What happens on the day that is not ordinary?

If there is no room for it, the plan works only while the day stays ordinary.

HANDS ON

This week, at home

Pick one plan at home — a water tank, a cooking-gas cylinder, a first-aid box. Ask what happens the day it runs out sooner than usual.

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The same gap, three costumes

Dhruv walks toward the school gate on a quiet morning, backpack on, with a small satisfied smile, a few minutes ahead of an ordinary pace. — THE NEXT MORNING · Dhruv left a few minutes before he needed to.

The same gap runs through the bus, the phone and the tin box. Design for the error you know is coming, not for the day nothing goes wrong.

None of the three was careless. Each planned for the day they knew. The room was for the day they did not.

Dhruv did not know which morning would be the busy one. He was ready before it came.

Papa’s twenty minutes were right for most mornings. The few minutes Dhruv added were for the other one.

The room is there before you need it. That is the whole idea.

the same gap, three costumes

the usual journey

a few minutes

The same shape, three costumes. In each one, something was kept back before the day that needed it, and the day landed inside it.

A SCHOLAR INDIA REMEMBERS

Dhruv wished he could ask Uncle Dharmakirti one question: would you notice, before the day comes, that your plan has no room in it? He thought about the bus, and started leaving five minutes earlier.

Dharmakirti

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The name

People who design things for a living have a name for this: a margin of safety. A margin is room kept back on purpose. Safety is what the room is kept for: the day that has not come yet. Together, the two words say what Dhruv learned about his mornings.

You have seen the shape six times now. The words are new. The shape is not.

the name, one word at a time

what an ordinary day uses

margin: room kept back on purpose

The shape with no story in it. The margin is the band kept back. Safety is the day it is kept for, landing inside it.

A LITTLE HISTORY

Where you will see this next

Next chapter: an alarm that goes off for a real reason, and one that keeps going off after the reason is gone. Telling the two apart is a different question from this one.