Because the two faces of the puri do not set at the same instant. The face that hardens first can no longer stretch; the face that is still soft is the one the steam pushes out, so it spreads over a larger area and becomes thin.
Follow what happens inside the puri:
The rolled disc holds water bound in the dough.
Hot oil is at roughly 180 °C, far above the boiling point of water. Within a second or two the outer surface of the disc dries and stiffens into a thin, sealed crust.
Under that crust the water flashes into steam. The same water needs more than a thousand times as much room as steam, so the pressure inside shoots up.
The dough underneath is still soft and stretchy, so instead of bursting it splits into two layers and balloons out.
The layer that met the oil first — the one we also hold down with the slotted spoon — has already stiffened, so it stretches very little. The other layer is still soft, gives way, and stretches. The same amount of dough spread over a bigger area must be thinner.
Cross-section of a puffed puri. Steam trapped under a sealed crust splits the disc into two layers; only the softer layer can still stretch, so it thins out.
Why it happens: rolling out a disc by hand never gives a perfectly even thickness either. The thicker patches hold more water and stay soft longer, so they are the parts that give way when the pressure builds. Thickness, temperature and how the puri enters the oil together decide which face ends up thin.
Check it yourself: roll two puris of exactly the same thickness. Slide one in flat and let the other fall in edge-first. Is the thin side always the side that touched the oil last? Do this only with an elder beside you.
Did you know? The book says this plainly on page 7 — the puffing of a puri is still not completely understood by scientists. The explanation above is our best account, and the fine details are an open research question sitting in your own kitchen.
Q2.
Are there more grains of sand on all the beaches and deserts of the world, or more stars in our galaxy?
Answer
There are far more grains of sand — roughly a billion times more. Our galaxy is enormous, but a star is enormous too, so there is room for only a few hundred billion of them. A sand grain is tiny, so a single beach already holds an uncountable number.
You can settle this yourself with an order-of-magnitude estimate — you only need the power of 10 to be right.
Take one grain ≈ 0.5 mm across → volume ≈ 0.125 mm³
1 cm³ = 1000 mm³ → 1000 ÷ 0.125 = 8000 grain-volumes
Grains never pack perfectly; about 60% of the space is filled
So 1 cm³ ≈ 8000 × 0.6 ≈ 5000 grains
1 m³ = 106 cm³ → 1 m³ ≈ 5 × 109 grains
Sand on all the world’s beaches (a widely used estimate) ≈ 7.5 × 1018 grains
Add the deserts — the Sahara alone dwarfs every beach — total ≈ 1020 grains or more
Stars in the Milky Way, our galaxy ≈ 100 to 400 billion = 1 to 4 × 1011
1020 ÷ 1011 = 109 → about a billion grains of sand for every star in our galaxy
Why it happens: the two numbers feel similar because both are “huge”, but huge is not one size. Every step of ten multiplies the count, and sand beats stars by nine such steps. Our brains have no feel for that gap, which is why the question is worth asking.
Did you know? Change one word and the answer flips. If you compare sand with all the stars in the observable universe — about 1022 to 1024, because there are roughly two trillion galaxies — the stars win easily. That is the whole lesson: a scientific question must state exactly what is being compared.
Q3.
Right from Grade 6, we’ve observed the incredible diversity of plants and animals around us. From the different shapes of leaves to the many kinds of insects — why has nature created such a vast variety?
Answer
Because there is no single design that works everywhere, and living things are not built to a fixed plan. Offspring always vary a little from their parents; in each place the variations that suit that place survive better and get passed on. Over an immense stretch of time this has filled every corner of the Earth with a different kind of life.
Two ideas do all the work:
Every habitat sets a different problem. A desert demands saving water. A pond demands staying afloat. A dark forest floor demands catching very faint light. One body plan cannot solve all of them at once.
Variation is inherited. No two seeds of the same plant, no two grasshoppers, are identical. In a dry place the individuals that happen to lose less water live longer and leave more offspring, so that feature becomes common there. Populations living in separate places drift apart, and after a very long time they are clearly different kinds of organisms.
Habitat
Problem it sets
Feature that answers it
Desert
Water is scarce and the Sun is fierce
Cactus: leaves reduced to spines, thick green stem stores water
Pond surface
Must stay afloat and reach light
Water hyacinth: spongy, air-filled swollen stalk
Dark forest floor
Very little light gets through
Broad, thin, large leaves that catch every ray
Cold, windy mountain
Snow load and freezing winds
Pine: narrow needle-like leaves on a conical tree
Why it happens: organisms are also part of one another’s surroundings. A flower’s shape suits a particular insect’s mouthparts; a leaf turns bitter against the caterpillar that eats it; the caterpillar in turn tolerates the bitterness. Each change pushes another change, and the variety multiplies far beyond what the physical environment alone would produce.
Tip: in science, “why” means “how did this come about”, not “for what purpose”. Nature did not sit down and design variety; variety is what you get when inherited variation meets many different surroundings over a very long time. Scientists call this process natural selection.
Q4.
Is there such a question that makes you curious about the world? Write it here! _________________________________
Answer
Write your own question in the blank in your book — but make it a question you can actually investigate. The chapter is not asking for a clever-sounding question; it is asking for one that can start an experiment.
A good question for this page passes three tests:
It is about something you can see or do — in your kitchen, courtyard, school ground or on your way home.
It is specific. “How does nature work?” cannot be tested. “Does curd set faster in a steel bowl or an earthen bowl?” can.
It can be written as “What happens to ___ if I change ___?” If you can fill both blanks, you already have an experiment.
Sample answers (adapt one, or better, write your own):
Why does a banana peel turn black in the fridge but not on the table?
Does water in an earthen matka cool faster on a windy day than on a still day?
Do the ants in my courtyard always take the same path to a spilt drop of sugar syrup?
Why does rangoli powder look brighter on a damp floor than on a dry one?
Does a phulka puff faster on a gas flame or on a hot tawa?
Try this: keep a small “question notebook” this year. Write the date, the question, and what you did to find out. By the end of Grade 8 you will be able to see which of your own questions actually got answered — and that is exactly how a scientist works.