NCERT Solutions for Class 8 Science Chapter 1 Exploring the Investigative World of Science (PDF) – 2027

Because the two faces of the puri do not harden at the same time. In hot oil the water in the dough turns into steam. The surface quickly sets into a crust, so the steam is trapped and pushes the dough apart into two layers. The face that sets first can hardly stretch. The face that is still soft is stretched by the steam over a larger area, so it becomes thinner.

The book itself says (page 7) that scientists do not yet fully understand how a puri puffs, so this is the best explanation we have.

There are far more grains of sand. Our galaxy has about 100 to 400 billion stars. Scientists estimate that the beaches of the world alone hold about 7.5 × 10¹⁸ grains of sand (7.5 billion billion), and the deserts hold much more. So there are many millions of sand grains for every star in our galaxy.

Because living things live in very different surroundings, and no single kind of body suits them all. Deserts, ponds, forests, mountains and seas each need different features. Young ones always differ a little from their parents. Those whose features suit their surroundings survive better and pass those features on. Over a very long time this has produced the huge variety of leaves, insects and other living things we see.

Write your own question in the blank — choose one that you can actually test. It should be about something you can see or do, and it should be specific.

Sample answer: Does curd set faster in a steel bowl or in an earthen bowl?

Dough rises because a gas, carbon dioxide, is produced inside it and gets trapped. In fermented dough (bread, bhatura, idli–dosa batter), tiny living things such as yeast feed on the sugar and starch of the flour and give out carbon dioxide. In dough made with baking soda, the soda gives out carbon dioxide when it meets an acid such as curd or lemon juice. Kneaded dough is stretchy, so the gas stays inside as tiny bubbles and the dough swells.

Yes. Thermometer records from land and sea, and measurements from satellites, show that the average temperature of the Earth’s surface has risen by about 1.1 °C to 1.3 °C since about 1850. The main cause is human activity: burning coal, petrol, diesel and gas adds carbon dioxide to the air, and this gas traps part of the heat the Earth gives out, like a blanket. Shrinking glaciers and rising sea level are further evidence.

To stay healthy, our body needs:

• a balanced diet (nutritious food)
• safe drinking water and clean air
• regular exercise and physical activity
• enough sleep and rest
• personal cleanliness (hygiene)
• protection from diseases, for example through vaccines.

Each of these keeps some part of the body working properly.

We fight infections in three ways:

1. The body’s own defences — skin, mucus in the nose, acid in the stomach and white blood cells stop or destroy germs.
2. Medicines — for example, antibiotics kill bacteria (they do not work against viruses).
3. Vaccines — they prepare the body in advance, so it destroys a germ before it can make us ill.

Best of all, we prevent infection: wash hands with soap, drink safe water, keep food covered, and do not let water collect where mosquitoes breed.

Yes. Each unit of a calendar is a count of a motion in the sky.

These motions repeat regularly, so by counting them people could predict the seasons, the time for sowing and the dates of festivals. That is how the first calendars were made.

A puri puffs up because the water in the dough turns into steam that cannot escape. In hot oil (or on the flame, for a phulka) the surface quickly hardens into a sealed crust. The steam trapped inside needs much more space than the water did, so it pushes the soft dough apart into two layers and blows it up like a balloon.

One side is thinner because the face that hardens first hardly stretches, while the softer face is stretched over a bigger area.

We make two lists — what we can change or control, and what we can observe or measure:

Change only one thing at a time and keep all the other conditions the same.

We should note down whether the oil splattered, smelled or smoked, because these are observations too. Splattering shows that water reached the hot oil; a smell shows that the oil or bits of flour in it are burning; smoke shows that the oil is too hot. Such notes help us explain why a puri did or did not puff.

Usually, fresh dough rested for about 15–20 minutes puffs best — but we should find out with a fair test. Fresh dough still holds its water, which becomes the steam that puffs the puri. Stored dough dries at the surface, and dough kept for many hours may turn sour.

Fair test: make puris from fresh dough and from stored dough, keeping the flour, thickness, size, oil temperature and way of dropping the same. Fry at least 5 of each and record whether each puffed (yes/no) and how many seconds it took.

The puri will puff up very little or not at all; it fries flat and crisp, more like a papdi or mathri. A puri puffs only when steam is trapped inside a sealed crust. The hole lets the steam escape, so no pressure builds up to push the two layers apart.