Answers to Last Issue's Do You Know?

1. What is fire and how does it burn?

Answer: Suppose cooking gas escapes from a gas stove. It mixes with the air, but nothing much happens. But then, a very small spark sets off a chain reaction resulting straight away in a hot flame. What causes this? Where is this huge difference?
Sparks have enough energy to break a few molecules into pieces. We call them free radicals and they react billions of times faster than the fuel and air gas molecules. When they react, they give out lots of heat and light producing a flame. That flame can produce more radicals and spread to other things starting a fire. When a solid light log of wood is heated perhaps with a flame, some of it turns to gaseous fuel which mixes with air and it too starts to burn.
As long as there is oxygen fuel and heat, a fire once ignited can burn indefinitely. The world record for the longest burning fire is a bed of coal 30 metres underground beneath Mt. Wingen in New South Wales, Australia. It is still smouldering, thought to be ignited by a lightning strike, heating the top of the coal over 5,000 years ago.

2. Do our actions affect our genes, and if yes, how?
Answer: Our actions do surely affect our genes, and in different ways. The difficult part is being precise about which ones and how.

If you train regularly in physical exercises, what happens to your muscles? They get bigger, don't they? This means that they are growing. Then they must be producing more of the tissue in a muscle that makes you strong. These are the contractile filaments, actin and myosin. Those are proteins and they get made by turning on a gene that tells our cell how to make them. 
Thus, in response to your activity, you trigger more genes to get turned on in your muscle cells to make more of the bulk of your muscle. This is one good example of immediately how one of your actions affects your DNA. 
Also, we know for a fact that stresses and strains cause your brain to change its shape and put out connections from one set of cells to another. Those are controlled by genes and in response to certain long term stresses, certain cells increase or decrease the production of certain nerve transmitter chemicals. Again, this is a change in response to your activity.
Recent research speaks of another phenomenon called epigenetics. In this, the DNA itself does not change, but some chemical markers added to the outside of the DNA, almost like signposts that can turn genes on or off, or turn the amount that the gene is turned on up or down, and this has an effect.

3. How many trees should we plant so we can combat climate change?
Answer: To answer this, we first need to figure out how much carbon is stored in the short, medium and long term in forests. In general, when you have enormous trees, the wood and the carbon inside them is carbon that would otherwise be in the atmosphere.
It is thought that about half the dry biomass of trees is carbon. That is, if you take the water out of a tree, then half the rest is carbon. Then, a tonne of carbon stored inside a tree roughly translates to about 3.6 tonnes of carbon dioxide in the atmosphere. We are told that last year alone 36 billion tonnes of carbon dioxide were put into the atmosphere just from burning fossil fuels. So, it's a lot of trees.
A few years ago, there was a study based on emissions only in Britain. According to it, if we wanted to offset just the emissions from cars in the UK, we would need to plant between 3 and 12 million hectares of woodland, depending on which species because that matters. Currently, there's only 2.5 million hectares of forest in the UK and 23 million hectares of land. If we were to offset all emissions in the UK, we would basically need 2 Britains. There is simply not enough space. We would need to plant about 50 million hectares of conifers in plantations to fix that carbon.
There is another way to look at this. There are a lot of trees in the world and a lot of carbon is fixed inside those forests. So we must also try and stop cutting down these since doing so releases a huge amount of carbon dioxide into the atmosphere. Also note that mangroves, which are trees that grow at the edge of the oceans, are great stores of carbon. So we should be thinking about replanting those areas of habitat as well.
One more interesting point: if we consider the photosynthesis that grabs carbon dioxide from the atmosphere and makes it into organic molecules, only one half of that happens on land in forests and grasslands etc. The other half happens in the ocean. Moreover, carbon dioxide just dissolves in the ocean. So the oceans are enormously important as well.

4. Why are planets round? Why not other shapes?
Answer: This is about gravity being a central force. It acts radially inwards and pulls anything with mass towards its own centre of mass. To go back to the formation of planets, we expect that stars and planets would have condensed and collapsed down from massive clouds of gas and dust, so-called nebulas. As they were collapsing, they were attracting other bits of matter. Then the natural equilibrium state is for everything to be as close to that centre as possible and to be in a sphere. It wants to be equidistant in one sense. Bigger objects have more mass and that strength of gravity is greater. So, it really starts to smoothen out the surface. A smaller object might be a cube and material properties of that cube can keep it up without gravity pulling it into a sphere.
It is a little like what happens with raindrops. As a droplet comes down, it forms a sphere, but then with the air current pushing it, it smears out a bit. This is a similar phenomenon: the water sticking, pulling itself together.

5. I saw an advertisement for heat sensitive paint. Is this true? How does it work?
Answer: Yes, heat sensitive paint exists. The way it is done, the paint is mixed with a black dye that has crystals of a salt which is either very acidic or very alkaline, and that paint is actually pH sensitive. It is sensitive to whether the environment is acid or alkaline. As the temperature changes, that material melts and mixes with the dye. This actually changes the optical properties of that dye and changes it from being very opaque black paint to being completely transparent. So, it is all governed by the melting temperature of these crystals that you have mixed in with that dye that change the pH of the environment very radically to a particular temperature. 
Suppose that you have a tea mug coated with the black dye. Underneath the black ink, they have got a picture which is revealed at certain temperatures when that black ink becomes transparent. When the mug is hot, the dye is black, you cannot see the picture and then as you drink your tea, you put down the mug, the mug cools down and the picture is revealed from the top. 
With the crystals in there, they can change between liquid state and solid state, mix in with that dye many, many times and that mug can carry on changing as many times as you want it to.

6. From how far can flies smell food?
Answer: The answer is surprisingly precise. Scientists have established that flies can smell meat from a distance as far as 7 kms away, although only under very, very favourable conditions. Mark-release-recapture experiments performed with house flies caught at a poultry farm showed that flies could find their way back to the farm from a distance of up to 7 km. 
To get this data, scientists had to tag and release 160,000 flies of which only 0.05% were recaptured. So, flies can do it but only a few manage this feat. 7 km is not only a considerable distance to smell something. For a tiny fly, even travelling 7 km is a major effort, taking several days. Even so, flies are able to detect a smell of rotting meat over long distances. The exact distance depends on many factors such as wind conditions, size of the meat and landscape features.
For flies to smell something 7 km away, it nevertheless has to be a substantial piece of meat like a herd of dead elephants rotting away in the sun or an entire poultry farm. How do flies manage this? Well, it involves no magic - simply, a very good sense of smell. Flies like all other insects detect odours with their antennae which are densely coloured hair-like structures containing olfactory sensory nerve cells. These nerve cells are extremely sensitive to volatile chemicals, odours that is. In the case of flies that feed on the dead and decaying flesh, so-called Carrion flies, their olfactory cells are also optimised towards 
detecting odours of rotting corpses.