Answers to last issue's Do You Know?
Source: {\it New Scientist} Forum

1. I have heard of the dance of bees, but how do ants know to gather in large numbers when one of them, presumably, has discovered a tasty bit in our kitchen?
Ans: Ants forage randomly. On their way back to the nest with food they leave a pheromone trail that other ants will randomly encounter and identify as "pointing" to a food source, according to the concentration of pheromone at each point.
So the pathfinder forager ant leaves pheromone trails from the spot where the tasty morsel is residing in your kitchen all the way back to the colony - hence it is able to organise a nice foraging party. Other ants follow the pheromone trails and also reinforce it, but once all the food is consumed, they stop leaving pheromone trails and disappear from your kitchen floor.
One in five foragers are pathfinder ants that can detect previously marked pheromone paths for up to 48 hours. 
Perhaps the best way to avoid this is to clean your kitchen daily.

2. What would we see if we had glasses that converted sound waves into light waves?
Ans: The difference between light and sound is the wavelength and frequency. The audible sound spectrum is 20-20,000Hz. Visible light spectrum is 400-790THz.
Suppose that have an equipment that amplifies the frequency of sound so that it appears in the visible spectrum. This would be something like a mapping of light for each audible frequency of sound. 
Now if you visualise this apparatus, it would show points of light for each sound wave hitting a set of microphones. Because the source is amplified many times also assume electronic noise in the amplification process. The result will be like taking a 4X4 pixel image and magnifying it to 40mX40m pixel image. The human brain will not be able to resolve the contrast in order to identify objects as we see today. That is, sound reflected from a chair will not form an image of a chair; but based on the property of the chair (wooden or metallic) will reflect sound with low or high frequency.
What our brains will probably see is blobs and bands of colour based on high and low frequency. When somebody hits the bass drums or knocks on the floor with their boot we will see Red. A child's shrill scream will show as blue and a mixture. A room full of wooden chair will show red bands, a room full of metal chairs will show violet bands. The dancing and dynamic display of colours would be parallel in many ways to the sound it's converted from, just like audio visualisers software. Intensity, frequencies, structure of waves, these all could easily be portrayed in the form of light. 
The intriguing question is: could we get accustomed to it by such a degree that we could communicate through these sound-lights? Right now, we can only say we don't know. But there are stories of colour-blind people who have learned to *hear colours*, so why not see sounds as well?

3. What causes or inhabits the black cracks that form on old, infrequently used bars of soap?
Ans: It is a mould called Aspergillus niger.
Bar soap is essentially the sodium salt of a fatty acid, which, like table salt, absorbs moisture. Even a brand new bar is hydrated to some extent, being kept so by its impermeable wrapper and subsequent everyday wetting. But if it is exposed to the air for long periods, it dries out and shrinks, causing its surface to crack. Drying mostly afflicts older bars that have worn small, because when soap becomes too small to use we don't throw it away immediately but leave it aside.
The mould A. niger is ubiquitous in soil and its spores are readily dispersed in air, where still, indoor conditions allow them to settle. Soap is typically left in rooms where humidity builds up from hot water usage. Any condensation or splashes accumulate in the soap's cracks, these recesses being slowest to dry. Coupled with cosy indoor temperatures, the mould is encouraged to start growing, in the form of vegetative "hyphae" that penetrate the soap for fatty nutrition and the visible reproductive film above.
Does this affect our health? Though A. niger in high doses can cause bad reactions in people, in small amounts it seems not to cause problems. On the other hand, it has many beneficial uses in food production and medicine. It is used to find out micronutrient content in soil for agriculture. It is also used by clinical researchers to evaluate anti-fungal treatments.
Most interestingly, one product of A. niger, gluconic acid, is itself used in cleaning agents!

4. Why is it that on the surface of an apparently homogenous potato a new sprout bursts forth? Is there something special at that point?
Ans: The surface of a potato may seem more or less homogenous, but close inspection, especially of a potato that has not been exhaustively scrubbed clean, will reveal small structures known as "eyes". These are buds from which the potato (which is a swollen, underground stem of the plant) will sprout.
At one end the potato spud retains the scar or stub of the stem that connected it to the mother plant. At the other is a central bud with other buds arranged around it in a rough spiral. These are ready to grow in the right season and conditions. Since the daughter tubers grew roughly radially outwards from the mother, the next generation will spread outwards a little further reducing competion. Taters are clever little things really. Gardeners know the end with the buds as the rose end, and attempt to plant the tuber with it facing upwards.
Scientists say that the spud is actually a stem tuber, so technically potato is not a root crop, and the buds are akin to those on a stem.

5. My birthday cake had some "trick" candles on it that I couldn't blow out, however hard I tried. How do these work?
http://www.last-word.com/content_handling/show_tree/tree_id/5820.html

A "trick" candle is like that. You blow it out and it 'magically' re-lights itself in a few seconds, usually accompanied by a few sparks. 
The difference between a normal candle and a trick candle is what happens just after you blow it out. When you blow out a normal candle, you will see a thin ribbon of smoke rise up from the wick. This is vapourized paraffin (candle wax). The wick ember you get when you blow out the candle is hot enough to vapourize the paraffin of the candle, but it isn't hot enough to re-ignite it. If you blow across the wick of a normal candle right after you blow it out, you might be able to get it to glow red-hot, but the candle won't burst into flame on its own.
Trick candles have a material added to the wick that is capable of being ignited by the relatively low temperature of the hot wick ember. When a trick candle is blown out, the wick ember ignites this material, which burns hot enough to ignite the paraffin vapour of the candle. The flame you see in a candle is burning paraffin vapour.
What substance is added to the wick of a magic candle? It's usually fine flakes of the metal magnesium. It doesn't take too much heat to make magesium ignite (800? F or 430? C), but the magnesium itself burns white-hot and readily ignites the paraffin vapour. When a trick candle is blown out, the burning magnesium particles appear as tiny sparks in the wick. When the 'magic' works, one of these sparks ignites the paraffin vapour and the candle starts to burn normally again. The magnesium in the rest of the wick doesn't burn because the liquid paraffin isolates it from oxygen and keeps it cool.

6. Why is it warm in the hay? Can it actually catch fire on its own?
Ans: The answer to the second question is, perhaps surprisingly, yes.
Hay is one of the more studied materials in spontaneous combustion. As hay varies by the type of grass and location grown utilized in its preparation, it is very hard to establish a unified theory of what actually occurs in hay self heating. It is anticipated that dangerous heating will occur in hay that contains more than 25% moisture content. The largest number of fires occurs within 2 to 6 weeks of storage, with the majority occurring at 4 to 5 weeks.
How does this happen? The process may begin with microbiological activity (bacteria or mould), but at some point, the process has to become chemical. Microbiological activity will also limit the amount of oxygen available in the hay. Moisture appears to be quite important, no matter what process. At 100 degrees C, wet hay absorbs twice the amount of oxygen of dry hay. There has been conjecture that the complex carbohydrates present in hay break down to simpler sugars, which are more readily oxidized.
While we are thinking about this, here is a question: do you know why we talk of searching for "a needle in a haystack"? Do you think people used to drop needles in haystacks and search for them? Actually they did use needles -- to check how hot the hay was!
A hay needle or rick needle, is one of the instruments, being used in rural areas in Britain. It is pushed inside the haystack, and when pulled out, used to smell the colour and mosture content of the hay. The end is approached with care, and it seems to radiate heat. There is a mercury thermometer in the end, and if the reading was 71 degrees C or above, then it was considered a fire risk. If the temperature measured was below this, and when measured over time showed a downward trend, the haystack would be left as it was.