Answers to last issue's Do you know?

1. How do oysters make pearls?
Ans: Most jewelry is fashioned out of precious metals and jewels that are found buried in the Earth, but pearls are found inside a living creature, an oyster. Pearls are the result of a biological process -- the oyster's way of protecting itself from foreign substances.
Oysters are not the only type of mollusk that can produce pearls. Clams and mussels can also produce pearls, but that is a much rarer occurrence. Most pearls are produced by oysters in both freshwater and saltwater environments. To understand how pearls are formed in oysters, you must first understand an oyster's basic anatomy.
Oysters are bivalves, which means that its shell is made of two parts, or valves. The shell's valves are held together by an elastic ligament.  This ligament is positioned where the valves come together, and usually keeps the valves open so the oyster can eat.
As the oyster grows in size, its shell must also grow. The mantle is an organ that produces the oyster's shell, using minerals from the oyster's food. The material created by the mantle is called nacre. Nacre lines the inside of the shell.
The formation of a natural pearl begins when a foreign substance slips into the oyster between the mantle and the shell, which irritates the mantle. The oyster's natural reaction is to cover up that irritant to protect itself. The mantle covers the irritant with layers of the same nacre substance that is used to create the shell. This eventually forms a pearl.
So a pearl is a foreign substance covered with layers of nacre. Most pearls that we see in jewelry stores are nicely rounded objects, which are the most valuable ones. Not all pearls turn out so well. Some pearls form in an uneven shape -- these are called baroque pearls.
Pearls come in a variety of various colors, including white, black, gray, red, blue and green. Most pearls can be found all over the world, but black pearls are indigenous to the South Pacific.

2. How does velcro work?
Ans: The hook-and-loop fastener was invented in 1941 by Swiss engineer George de Mestral who lived in Switzerland. The idea came to him one day after returning from a hunting trip with his dog in the Alps mountains. He took a close look at the burrs (seeds) of burdock that kept sticking to his clothes and his dog's fur.
He examined them under a microscope, and noted that they were made up of hundreds of "hooks" that caught on anything with a loop, such as clothing, animal fur, or hair. He saw the possibility of binding two materials in a simple fashion, if he could figure out how to duplicate the hooks and loops. He also had to make sure the materials could later be separated as well. It was essentially an alternative to the common zipper.
He settled on nylon as being the best synthetic material, since cotton lost its "stickiness" very fast. The nylon hooks were stitched to the base fabric with hot infrared light.
It took a long time to make the loops: in a usual hook-and-eye method, the hooks and eyes are matched. In this case, there were so many hooks that it was difficult to make matching loops. The final solution was a master-stroke: the loops were first made. Then half the strip of loops was taken and the tops of the loops were trimmed off. These then took the form of hooks that perfectly matched the remaining half-strip of loops!
When the two sides are pressed together, the hooks catch in the loops and hold the pieces together. The strength of the bond depends on how well the hooks are embedded in the loops, how much surface area is in contact with the hooks, and the nature of the force pulling it apart.  Velcro is strong enough that a 5 cm square piece is enough to support an 80 kg person.
Special velcro fasteners made of Teflon loops, polyester hooks, and glass backing are used on space shuttles as well.

3. What are the biggest cells in the human body?
Ans: Neuron cells are the core components of the nervous system, which includes the brain and spinal cord.
A typical neuron possesses a cell body (often called the soma), dendrites, and an axon. Dendrites are filaments that arise from the cell body. An axon is a special filament that arises from the cell body and can be even 1 m long in humans. However, although they are the longest cell, they are only few microns thick.
The anterior horn cell is a motor neuron in the spine. It is the largest cell in the body, about 135 microns.

4. How many times can you fold a piece of paper?
Ans: Suppose it is an ordinary piece of paper, about 20 cm square (it's not important for it to be square, only convenient). Suppose you fold it in half, and then again in half from the other side. You get a square of half the size (each side is halved). So every two folds halves the size of the paper.
Now every fold doubles the number of layers: the first fold has two, the second four, etc. Let us assume that we have ordinary paper, about 0.1 mm thick. So if you have 4 folds, the thickness is 2^4X0.1 mm = 1.6 mm, which is not so bad. But once you have 10 folds, the thickness becomes 2^10X0.1 mm = 102.4 mm = 10.24 cm, which is a pretty fat object! 
When the number of folds is such that the thickness of the folded paper is about the same as its size, it becomes stiff and difficult to fold.  When does this happen?
Fold  Size (cm) Thickness (mm)
0	20 	0.1
2	10	0.4
4	5	1.6
6	2.5	6.4
8	1.25	25.6
10	0.625	102.4
You can see that after about 8 folds, the size of the paper is 1.25 cm square, while the thickness of the bundle (with 2^8=256 layers) is 25.6 mm or 2.56 cm. So it is thicker than it is wide and will be extremely thick and difficult to fold.
If you take bigger and bigger sheets of paper, you can probably reach about 12 folds, but it's very difficult to push beyond this.

5. What is house dust mostly made of?
Ans: There are many components of house dust, depending on which room you are in, and what kind of furniture it has.
House dust is made up of mainly fabric fibres (from clothes and bed sheets, and also if the sofa has lots of fibre). It also has bits of dry stuff like food or cardboard, untreated wood and good old fashioned dirt. There's also a lot of skin that people shed regularly. That's because skin cells keep flaking off without our noticing it. In fact, human skin cells are almost totally replaced every month!
Dust can also come in with wind: for example, dirt, bacteria, pollen, pollutants, etc. Dust mites settle in this dust as well, to scavenge off the skin cells, decomposing insects and bacteria.
Dust mites are not generally visible, except with a microscope; they are 200-300 microns long, they eat dead skin cells and live in bedding, carpets and soft furnishings. Many allergies are caused by them.
Outdoor dust is composed of fine particles of soil and stone dust, as well as pollen and other plant material.