Answers to Last Issue's Do You Know?
1. There are many household things made of plastic in our homes that are non-recyclable. How can we best dispose of them? Do we bury then, burn them, what is best to do?
Ans: Plastic is a petroleum derivative and difficult to dispose of. By indiscriminately dumping plastic into rivers, oceans and dumping grounds humanity has created a negative impact on the environment. 
On the other hand, plastic is immensely useful, and being cheaper than material such as wood and metal, has eased the lives of the poor. If recycled, it can be used well. Plastic can be made to form recycled polyester which in turn is made to produce a number of things like shoes, T-shirts, bags, chairs etc. thereby causing less strain on natural resources.
Very few people know that every type of plastic can be recycled several times. Nearly a quarter of the plastic being produced is currently not recycled. This majorly is due to items like wrappers, plastic bags, milk pouches, multilayer chips packets etc. They are not picked up for recycling only because they are light in weight and dirty. These plastics lie in the dump yards and cause havoc. 
The plastic recycling process includes cleaning of the plastic, cutting it into flakes and then melting it at high temperature for moulding into desirable useful form.
Collection: The first step is to separate Dry and Wet waste. The former can be emptied and cleaned easily and stored away, often sold to people who in turn sell plastic in bulk. The rest of the plastic which are mixed with wet waste, and take time and energy to be cleaned, go to the garbage collection trucks, eventually landing up in dumping ground.
Segregation: This is a crucial step, separating waste into different categories. Often different coloured bins are used. Green is generally used for biodegradable or organic waste, such as kitchen scraps and garden waste. Blue is typically designated for recyclables like plastic, paper, metal, and glass. Where used, red indicates hazardous waste, including batteries, e-waste, and medical waste. (In some societies, medical or infectious waste is handled separately, yellow being its colour.) Black or Gray is used for general waste that does not fit into other categories.
Cleaning and flaking: After the plastic reaches the recycler, all the plastic is sorted again, cleaned with large amount of water and then shredded into small pieces. The shredder then converts the plastic into flakes. Once that is done, the flakes are then subjected to further washing, drying and are ready to be shipped to manufacturing units for making several kinds of recycled products.
A range of products are now made from recycled plastic. Look around you, and ensure that all the plastic you see is reused and recycled.

2. It is summer and the air is full of flies. Where do they go in winter?
Ans: Flies do not really go anywhere for the winter. Where it is cold, like in northern India, the adult flies die off in outdoor spaces but the species survives the winter as larvae (also called maggots) or pupae — the shell in which they transform from larva to fly in the spring.
A favoured location for larvae and pupae to survive is under a pile of manure (or in India, cow dung!), but they can make do with your house. A house fly can live up to 60 days, but without food, they only survive about three days. Even if you are not handy with a rolled newspaper, the problem is easily solved by not leaving garbage lying around!
The real problem is that that are produced in large numbers: their cycle is opportunistic, flexible, and depends heavily on ambient temperature. Females lay about 500 eggs that take between 8 and 48 hours to evolve into larvae, which in turn take between 4 and 30 days to complete their development, depending on the temperature. During these immature phases, they live in decomposing organic matter, often excrement, which by the action of bacteria can ensure a relatively moderate temperature.
3. How do creams go through the skin?
Ans: The first thing to remember is the basic structure of the skin: it is designed to keep things out! The top layer, the epidermis, consists of a watertight roof known as the stratum corneum, where cells are glued together with fatty acids. The middle layer, the dermis, contains blood vessels and vitally important structural proteins collagen and elastin. This is the layer that makes your skin appear plump and youthful. The bottom layer, the hypodermis, stores fat and energy, insulating and protecting the body, connecting to bones and muscles. 
The thinner the skin, the easier it is for cream to penetrate. The skin of your face is thinner than the skin on your palms and the soles of your feet for example. The delicate skin around your eyes is the thinnest of all, so most vulnerable to sensitivities. 
There are two ways by which creams may go through the skin: absorption and penetration. Absorption refers to when an ingredient reaches the bloodstream, and we do not what cosmetics to do this! Penetration refers to when a chemical gets past the outer layer into the deeper layers of the epidermis. This is what creams do. Often they do not even penetrate into the skin, but only provide extra, hydrating layer on top of the skin, nourishing the skin barrier.
What about the creams that do enter into the three layers? There are three routes for this.
Intercellular: They slip in between the cells of the outer skin. The glue holding these cells together are fatty acids, so water or oils cannot use this route.
Intracecellular: This is when extremely tiny ingredients actually slip into the cells of the skin and then through them.
Follicular: This is when ingredients can enter through openings that already exist in your skin: think of your pores and hair follicles. However, these tiny openings are few and far between, so only a few creams can enter into the skin this way.
In general for chemical creams, the molecules need to be small to slip between the gaps in the skin cells and the ingredients should be able to dissolve in oils or lipids.
The condition of the skin also plays a crucial role in the absorption of creams. Hydrated, exfoliated, and treated skin can allow better penetration than dry, flaky skin. When skin is dry, it can form a barrier that prevents the ingredients from penetrating. 
What regular use of the chemicals in creams does to skin is another matter. 

4. If you have been observant, you would have noticed that boat and plane propellors are in different places. Why?
Ans: This is a very interesting question. There are good physics reasons for these different propeller placements, but also several practical ones.
The physics first. The big difference between water and air is that water is very much more dense than air. So if you take one metre cubed of air (1 m^3), it weighs about a kilogram, whereas a metre cubed of water weighs about a ton, a *thousand times* more. This means that propellers for aircraft need to be much bigger. This is because the thrust you get from a propeller is related to the amount of the mass flow that passes through the propeller and the speed with which the propeller can push that backwards. Thus it matters where you put propellers on an aircraft. 
Now the practical aspects: you cannot have a propeller that is too close to the ground that might then hit the ground. You cannot have it towards the back of an aircraft that might affect how the aircraft could land. You also do not want people sitting too close to propellers.
The distribution of weight on an aircraft is very important. The
propeller is also attached to an engine and the engine tends to be
heavy. If you have too much weight towards the back of an aircraft, that
will affect how the aircraft can be controlled in flight. So you prefer
to mostly have the aircraft propellor in the front, although some models
have them at the back.
When it comes to boats, because the water is a thousand times more dense than air, how the flow enters the propeller is important. The propeller on the boat is much smaller and we mount it at the back of the boat so that the flow is just leaving the boat as it passes the propeller.
Another difference is that there is air both inside and outside an
aircraft, so if the propellor joint/seal leaks a little, it doesn't matter. But there should be only air and no water inside a boat, so if the propellor joint leaks, you would rather place it at the back of the boat.
It is also a question of where the engine is mounted. We often want a small boat to pitch up a little bit so that the boat can then move more smoothly through the waves. The weight of the engine can help with that, so the relative placement of engine and propeller is crucial.
Researchers are now experimenting with electrically-driven propellers, which can made much smaller. Then they can be distributed over the aeroplane, giving more flexibility in the design of the aeroplane. Aircrafts in future may have small propellers mounted on the wings, blowing air over the wing to improve wing performance.