Answers to Last Issue's Do You Know?
1. In real life and cartoons, it is common to see people slide and fall on banana peels. What makes banana peel so slippery?
Ans: We’ve all seen the cartoons. Bugs Bunny wolfs down a banana and casually tosses the skin onto the floor. Moments later, Elmer Fudd comes racing in, steps on the banana peel and goes flying. The music plays, and Bugs Bunny wins the day again.
But why is the banana peel so slick? To answer this question, the scientists crushed fruit peels and looked at them under a microscope. They observed that, when crushed, banana peels produce a gel-like substance, which might be the source of the slippery results.
To determine exactly how slippery a banana peel is, Mabuchi, a biomechanics researcher, and colleagues used a total of 12 Cavendish bananas, the common yellow banana. They placed the banana skins interior-side down on samples of linoleum, a type of smooth flooring. The scientists stepped on five different sections of each banana peel, for a total of 60 measurements. The linoleum was placed on a force transducer, a sensor that measures the force, weight and pressure applied to an object.
The next step is literally forwards: a volunteer stepped carefully onto the banana skin and pushed his foot forward. By measuring the horizontal and vertical forces, the scientists measured the degree of "slipperiness" of bananas.
They quantified the "slipperiness" by measuring the coefficient of friction, which is a measure of the amount of friction between two surfaces in contact. The average coefficient of friction of a banana on linoleum was 0.066 (slightly higher, 0.083, for hardwood floor). This is very low, usually reserved for substances like Teflon (with a friction coefficient of 0.04), ice (around 0.05) or other well-lubricated surfaces. By comparison, rubber sliding on concrete has a frictional coefficient of 1.02. So it turns out that banana skins are, indeed, very slippery.
BOX (Untitled)
Banana peels are especially slippery, even when compared to the peels of other fruits, because of polysaccharide molecules in the peel. These chemicals are also present in the membranes where our bones meet. There is a similarity between the lubrication around cartilage and banana skin. They have similar friction. Understanding how they work could help with designing better prosthetics (artificial limbs).
END OF BOX
Kinds of friction
It might be worthwhile to distinguish between the different kinds of friction present in a banana-slipping scenario, suggests Blake Stacey, a statistical physicist at Brandeis University in Waltham, USA. When a person steps on a banana peel on the floor, there is dynamic friction between her forward-moving shoe and the top of the banana peel. There is also static friction between the bottom of the banana peel and the floor. For the person attached to the shoe to go flying forward, Stacey says, the dynamic friction must be larger than the static friction. While this is probably the case when someone is walking or running forward, the scientists did not differentiate between the two types of friction, which, Stacey notes, could result in “slightly overestimating the hazards” of the banana peel alone.
Any way, the next time you see a cartoon character go flying from a banana peel encounter, remember: It’s not just comedy, it’s real science.

2. What is the hardest substance in our body?
Ans: Tooth enamel is one of the four major tissues that make up the tooth in humans and many animals, including some species of fish. It makes up the normally visible part of the tooth, covering the crown. The other major tissues are dentin, cementum, and dental pulp. It is a very hard, white to off-white, highly mineralised substance that acts as a barrier to protect the tooth but can become susceptible to degradation, especially by acids from food and drink.
Enamel is the hardest substance in the human body and contains the highest percentage of minerals (96%), with water and organic material composing the rest. The primary mineral is hydroxyapatite, which is a crystalline calcium phosphate. Enamel is formed on the tooth while the tooth develops within the jaw bone before it erupts into the mouth. Once fully formed, enamel does not contain blood vessels or nerves, and is not made of cells.
The normal color of enamel varies from light yellow to grayish (bluish) white. It has been suggested that the color is determined by differences in the translucency of enamel, yellowish teeth having a thin, translucent enamel through which the yellow color of the dentin is visible and grayish teeth having a more opaque enamel.
The large amount of mineral in enamel accounts not only for its strength but also for its brittleness. Tooth enamel ranks 5 on Mohs hardness scale (harder than steel and less hard than titanium, which is one of the hardest substances known).
The high mineral content of enamel, which makes this tissue the hardest in the human body, also makes it demineralize in a process that often occurs as dental caries, otherwise known as cavities. Demineralization occurs for several reasons, but the most important cause of tooth decay is eating sugars and carbohydrates: cavities are caused when acids dissolve tooth enamel.
Did You Know?
The extent to which tooth decay is likely depends on factors such as how long the sugar remains in the mouth. It is not the amount of sugar ingested but the frequency of sugar ingestion that is the most important factor in tooth decay. Thus, eating a great quantity of sugar at one time in the day is less harmful than a very small quantity ingested in many intervals throughout the day. For example, in terms of oral health, it is better to eat a single dessert at dinner time than to snack on a bag of candy throughout the day.
Though enamel is described as tough, it has a similar brittleness to glass, so it can crack or break. May be you have fallen on a hard floor and chipped off your tooth? That's a bit of enamel coming off. It's very rare to find such a hard substance that can also crack and break. Of course, this happens rarely, partly because of the microstructure of enamel. So next time you put some hard food in your mouth and go "crunch", spare some thoughts of appreciation for your enamel that can withstand such pressure without cracking.

3. Can global warming and melting of glaciers affect gravity?
Ans: When people talk about rising sea levels due to global warming, maybe you though about water rising in a bucket when the tap is turned on: uniformly rising every where on Earth.
In fact, until nine years ago, many scientists also assumed the same thing. But there were reports of sea levels rising differently in different places. The scientists had predicted a sea level rise worldwide of about two millimeters per year in the twentieth century. This was found to be accurate on average, but there were large variations and scientists were trying to understand why. Then they realised that they needed to include gravity in their equations.
Ice in glaciers has a huge mass and hence exerts large gravitational pulls, so it pulls the nearby sea water towards itself. This causes sea levels to rise near glaciers. As the glaciers melt because of global warming, the amount of ice in the glacier decreases. So its gravitational pull decreases and nearby sea levels can actually fall!
Of course, everything depends on which ice sheets melt: there are glaciers at both the North and South poles and the result will be different if only one melts or both melt togeher. So Europeans would prefer only the North Pole ice to melt, while Australians would prefer the South Pole ice to melt!
But that’s not all. When glaciers melt, the redistribution of their mass causes changes in the earth’s rotational axis. The earth isn’t a perfect sphere. It’s flattened because it’s rotating. As the ice sheets collapse, the pole would move. For instance, if the South Pole ice in the West Antarctic sheet collapsed, the pole would move such that sea levels would rise even more in North America. 
Alarming though these projections may be, they are not immediate. It can take hundreds of years before this happens, and hopefully, all countries (and individuals) will help to reduce the impact of climate change.

4. Babies have 300 bones. But adults have only 206! What happened to the rest?
Ans: It may be difficult to imagine when looking at a tiny newborn baby, but that infant has around 300 bones — and those bones are growing and changing shape every day.
What are bones made of, anyway?
Even though bones appear to be tough and rigid, they’re actually made up of living tissue and calcium that’s always being built up and discarded throughout your life.
Most bones are made of several layers of tissue:
. periosteum: the thick membrane on the outer surface of the bone
. compact bone: the smooth, hard layer that’s seen in the bones of a skeleton
. cancellous: sponge-like tissue within the compact bone
. bone marrow: the jelly-like core of the bones that makes blood cells.
The process of bone development is called ossification. It actually begins when the embryo is around eight weeks old — pretty incredible! Even so, at birth, many of the baby’s bones are made entirely of cartilage, a type of connective tissue that is tough, but flexible. Some of the little one’s bones are partly made of cartilage to help keep the baby nice and flexible.
That flexibility is necessary so growing babies can curl up in the confined space of the womb before birth. It also makes it easier for mom and baby when it’s time for the baby to make the exciting journey through the birth canal during delivery.
As the baby grows into childhood, much of that cartilage will be replaced by actual bone. But something else happens, which explains why 300 bones at birth become 206 bones by adulthood. Many of the baby’s bones will fuse together, which means the actual number of bones will decrease. The space that separates the ends of two bones that eventually fuse is also cartilage, like the tissue you have in the tip of your nose.
The fusing of bones occurs throughout the body. You may notice that there are one or more soft spaces in between the bones in a baby’s skull. They’re called fontanelles, and they’ll eventually close as bones grow together. Replacing cartilage with fused bone begins when tiny blood vessels — called capillaries — deliver nutrient-rich blood to osteoblasts, the cells that form bones. Osteoblasts create bone that covers cartilage at first and then ultimately replaces it.
Also, bone growth in children occurs at the ends of many bones, which have growth plates. The growing tissue in each plate determines the final size and shape of the bone. When a person stops growing, the growth plates close. Growth plates are weaker than other parts of the child’s skeleton, and are therefore more susceptible to fractures and other injuries. This is why a fall off a bicycle may land a child in a cast, whereas adults can take a similar fall and just have a bruise — perhaps on your body as well as your ego.
By early adulthood, the fusing of bones and bone growth have stopped. Adult bones are very strong, but light. While they appear to be solid and unchanging, bones are constantly going through a process called remodeling.
Remodeling involves the formation of new bone tissue and the breakdown of older bone into calcium and other minerals, which are released into the bloodstream. This process is known as resorption, and it’s a perfectly normal and healthy part of bone function — in fact, it happens throughout life. But in children, new bone formation happens faster than resorption. So children need a lot of calcium to grow their bones.
The body doesn’t make calcium, so all the calcium you need has to be consumed in food or supplements. Healthy calcium-rich foods include low-fat dairy products (milk, cheese, yogurt), seeds, almonds, white beans, and leafy greens such as spinach. Weight-bearing exercises also strengthen bones. Finally, Vitamin D helps your body absorb calcium. Getting enough protein is also important for long-term bone and muscle strength.

Sources: Smithsonian Magazine https://www.smithsonianmag.com, Wikipedia, Harvard Magazine https://www.harvardmagazine.com, https://www.healthline.com