This is an open observation-based question — there is no single “textbook” answer, because the book itself tells us that scientists have not fully explained this phenomenon yet. However, based on careful observation and simple reasoning, we can build a good explanation:

1. When a flattened circle of dough is dropped into hot oil, the side that touches the oil first begins to cook and become firm a fraction of a second before the other side.
2. As the puri heats up, the water inside the dough turns into steam. This steam tries to escape and pushes the dough outward, making it puff up like a balloon.
3. Because the first side that touched the oil sets (hardens) slightly earlier, it offers more resistance to stretching, so it remains relatively thicker/stiffer. The side that is exposed to oil a little later is still soft and stretches more easily, becoming thinner as the steam pushes it outward.
4. The way the dough is slid into the oil (vertically, at an angle, or slowly) and the temperature of the oil can also affect which side cooks first and how much it stretches.

Key idea: This is exactly the spirit of the chapter — instead of memorising one fixed answer, we are encouraged to observe, ask “what if”, and design a small experiment to test our explanation (see the detailed worked investigation below).

This is a “thinking big” estimation question meant to spark curiosity about very large numbers — it does not need a precise calculation in Grade 8, but we can reason about it logically:

• Scientists estimate that the number of stars in just our own galaxy (the Milky Way) is about 100–400 billion (i.e., on the order of 10¹¹ stars).
• Separately, rough estimates of the total number of sand grains on every beach and desert on Earth are often quoted as being on the order of 10¹⁸–10¹⁹ grains.
• So, comparing only our galaxy’s stars with all of Earth’s sand grains, the number of sand grains is far greater than the number of stars in the Milky Way.
• (Interesting extension: if instead we compare sand grains on Earth with the number of stars in the entire observable universe — which is estimated to be far higher, around 10²²–10²⁴ — then the stars would outnumber the sand grains. The chapter’s question specifically says “our galaxy”, so within that comparison, sand grains win.)

The real purpose of this question is not the exact number — it is to make us appreciate how science helps us estimate and compare quantities that are too large to count directly.

This vast variety (biodiversity) exists mainly because of adaptation and evolution:

1. Living things exist in extremely different environments — deserts, forests, oceans, mountains, polar regions — each with different sunlight, temperature, water availability, and food sources.
2. Over long periods of time, organisms that develop features (shapes, sizes, colours, behaviours) best suited to survive and reproduce in their particular environment are more successful, and these useful features get passed on to the next generation.
3. For example, leaf shapes vary because different leaf shapes are better at capturing sunlight, reducing water loss, or shedding rain, depending on the climate. Insects show enormous variety because each species has adapted its body shape, mouth-parts, and behaviour to a particular food source, habitat, or way of avoiding predators.
4. This continuous process of organisms adapting to different conditions over generations is what produces the incredible diversity we observe in nature today.

This is a personal, open-ended prompt with no fixed answer — it is meant for you to fill in with your own curiosity. A good answer is simply any genuine “why” or “how” question about something you observe in daily life. Some sample answers a student could write:

• “Why do onions make us cry while cutting them?”
• “Why does a mirror reverse left and right but not up and down?”
• “Why do some fruits ripen faster when kept near a banana?”
• “Why is the sky blue during the day but red/orange at sunset?”

Any honest, observation-based question of your own is a correct answer here — the goal is to practise the very first step of scientific investigation: noticing something and wondering “why”.

Question to investigate: “What are the different things (factors) that may change the way a puri puffs up when fried?”

To test the effect of oil temperature (boiling hot vs. hot vs. not-very-hot) on puffing:

• Use dough circles of the same thickness for all trials
• Use dough circles of the same size for all trials
• Drop each dough circle into the oil in the same way (e.g., always sliding gently at the same angle)
• Only the oil temperature is changed between the three trials

This way, if the puffing result changes, we can confidently say it happened because of the change in oil temperature, and not because of some other hidden difference.

Besides numbers, a good investigator also writes down sensory observations during each trial, such as:

• Did the oil splatter?
• Was there any unusual smell?
• Did the oil smoke (sign of overheating)?
• Did the puri puff into a full balloon shape, or only partially?

After the first round of trials, new questions naturally arise, for example:

1. Do puris puff better when made from fresh dough or from dough stored for some time?
2. What happens if a small hole is pricked in the puri before frying?

This is the real nature of scientific investigation — each answer leads to new questions, and the process of observing, experimenting, and refining our understanding continues.

The puri puffs up because the water trapped in the dough turns to steam when heated, and this steam expands and pushes the dough outward like a balloon. One side tends to be thinner than the other because of small differences in how quickly each side sets (hardens) on contact with the hot oil, and how the dough is dropped in. The exact, complete physical explanation is still an open question — even scientists today don’t fully understand every detail of this everyday phenomenon — which is exactly the kind of real, unanswered puzzle this chapter wants you to get curious about.