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Plastic solution

Question: Checkout operators the world over vigorously rub any malfunctioning
credit and debit cards on the nearest available article of clothing. Does this
actually serve any useful purpose?

Answer: From my experience, a credit or debit card will fail to 鈥渟wipe鈥�
correctly for one of three reasons.

First, something has permanently interfered with the magnetic strip on the
card, so that the computer cannot read it. The cashier will have to type in the
number manually, and a new card will probably need to be issued. Secondly, the
machine is faulty and is unable to read the card.

However, the third reason the card cannot be read is the most common cause.
Dust or dirt of some sort collect on the magnetic strip. This obscures the
information from the electronic reader. A quick wipe on your sleeve is all that
is required to resolve this and, in the vast majority of cases, the card will
swipe successfully at the second attempt.

There is no great mystery and no big science behind this practice, at least
that I am aware of. If you keep your cards in the card compartment of a purse or
wallet, they should remain reasonably clean, and swipe easily on the first
attempt. This should also eliminate the first problem, because they will be
protected from anything that is likely to irreversibly damage the strip.

Charlotte Dadswell

Petworth, West Sussex

Answer: There is one drawback to rubbing the magnetic strip and it was
something I often experienced as a supermarket supervisor. Rubbing the card can
sometimes make it more difficult to read because it becomes charged with static
electricity that can interfere with the electronic reader.

The instinct to rub the card in an attempt to remove any dust that may have
stuck to it may work in the short term but the extra static charge the rubbing
has generated will ensure that even more dust will cling to the card later
on.

Cissy Azar

Sydney, New South Wales

It鈥檚 not

Question: Sorry, but I had to ask, why is nasal mucus often green?

Answer: Of all the body cavities in contact with the outside world, the nose
is probably one of the most hospitable: it is warm, very well aerated, moist and
supplies unlimited quantities of bacterial food secreted continuously by the
nasal mucosa (mucus contains quantities of glycoprotein and dissolved salts). In
other words it is an ideal breeding ground for bacteria, which are always
present.

Many of the common bacteria associated with humans are coloured,
Staphylococcus aureus is a golden yellow, for example, and Pseudomonas
pyocyanea (to give it its older, but more explicit name) is a shade of
blue.

Normally these and the multitude of other organisms that are inhaled
continuously into the nose are flushed out by runny mucus, which is swallowed.
The bacteria are usually digested.

However, if a situation arises where the flow of mucus slows down and then
becomes much thicker in response to an infection of any kind, then the bacteria,
in their ideal home, can multiply and produce the coloured mucus described.
This, as many parents know, is one of the less endearing characteristics of
babies and young children!

And, by the way, if you鈥檙e still wondering where the green colour comes from,
remember what happens when you add blue to yellow.

Laurie North

London

YELLOW FEVER

Question: How does the skin magically appear on top of the custard? I love
it.

Answer: The skin is dried custard. When custard is hot and freshly made,
water evaporates from the surface. The rest of the custard is also full of
water, but this is tightly bound to the carbohydrate in the cornflour and
protein in the egg, forming a gel.

As the surface starts to dry, it is unable to soak up water from the gel
below and forms a skin. Because this skin is kept hot by the custard below, the
water in it continues to evaporate until it is dry. In effect, the skin becomes
concentrated custard which is why it tastes so good.

In addition, the skin is a mixture of protein and sugar and, when kept hot,
this allows for a small number of Maillard reactions between the protein and
carbohydrate and also caramelisation of the sugar. These make the flavour even
better.

Tom Rees

Hove, East Sussex

This week鈥檚 questions

Hooded heritage: I have a problem which has been perplexing me for quite some
time. I was idly wondering how many of my ancestors were alive at the time of
Robin Hood. If a generation can be said to be about 25 years, and the
aforementioned Mr Hood allegedly lived in the 12th century, using this
approximate information there must have been 2³² (4 294 967 296) of my
forebears extant at the time.

The available breeding population of the areas in England and Ireland where
my ancestors originated, and where transport was limited, can only have been a
tiny fraction of this number, and I do not think that it is reasonable to say
that I am related as closely as this fraction implies to everyone from that part
of the world.

Can I really be amazingly inbred? Am I doing my sums wrong? Are everyone鈥檚
family trees so closely entwined in a great global hedge? Please advise.

Darcy Holt

Parkville, Victoria

What鈥檚 the crack? What causes the noise when you crack your knuckles or any
other joint?

Marty Brown

by e-mail, no address supplied

Through the hole: I recently did a parachute jump for charity and the one
disconcerting thing about the jump (apart from a fear of heights) was the large
hole at the top of the parachute. Why is it there? Does it help in any way to
reduce the drag on the chute?

Suzy Klein

London

Clear blue sea: Are the oceans blue, and if so, why? I鈥檝e heard that the
colour of the sea is determined by reflection from the sky but it appears blue
from outer space and from the perspective of deep-sea divers. And fresh water in
a swimming pool looks green. What are the absorption mechanisms involved? Is
pure water perfectly clear?

John Rodenburg

Cambridge

Pasta puzzle: If you bend a piece of dried spaghetti it breaks into three
pieces with the middle piece flying out. Why does this happen?

John Winn

Cambridge