The Byzantine emperor, a Christian monarch who lived in Constantinople, needed help.
He turned to the pope, who in 1095 called for all Christians to start a holy war against the
Suljuk Turks. Thousands rushed to join the Crusader armies. They crossed into Palentine
and recaptured the important cities of Nicaea and Antioch. Jerusalem fell in 1099 after a
desperate siege lasting six weeks, and the Crusaders took terrible revenge by slaughtering
thousands of Muslims. There were to be three more crusades: one in 1144, the second
in 1187 and finally the Children’s Crusade in 1212. Fifty thousand children set off from
France and Germany for the Holy Land. Many died on the journey, many more were
captured and sold as slaves in Africa.

Traditionally, glue is made from boiling up the bones, skins and other parts of animals such as fish
or horses in order to extract the gelatin, which is then purified and concentrated to make a sticky
substance that will form a bond as it dries. There are now many different types of glues, made from
plant material or minerals such as petrol, as well as different kinds of synthetic glues. One of the
simplest glues is flour-and-water paste and traditional wallpaper paste is made with starch. Latex
adhesives were originally made from the latex extracted from rubber trees, but synthetic forms are
now also available. Another type of synthetic glue is epoxy resin, in which the user mixes an adhesive
substance with equal quantities of a hardener. The two substances react chemically together, causing
the adhesive to set rapidly. Glues are widely used in many industries, such as furniture-making and food-
packaging.

There are a few factors that govern how valuable a stamp is to stamp collectors: the most important
is how rare it is. Obviously, very old stamps are uncommon so these, such as the ‘Penney Black’ very
seldom come up for sale and so are unbelievably expensive. Other things that might make a stamp rare
might be where a mistake was made in the printing and the stamp was withdrawn after only a few had
been sold. Undamaged stamps are more valuable than similar ones that have been damaged.

A cloud is made up mainly of water. When hot air rises from the land or the sea, it carries water vapour
with it. Because the atmosphere usually gets colder with height, and cold air can hold less vapour
than warm air, the vapor eventually condenses in the form of water droplets or ice crystals around
particles of dust to form a cloud. If it continues to cool further, the cloud will become denser until the
atmosphere is no longer able to hold the moisture and rain, hail or snow will fall. The general rule is that
the deeper a cloud is relative to the deeper a cloud is relative to the height of its base above the sea or
ground, the more heavily it will rain, hail or snow, so thin, high clouds produce no rain, while thunder
storms produce a great deal.

It would be easy to think that a duck’s feathers would soon become waterlogged so that it would sink,
but in fact, ducks, geese and swans, as well as seabirds, have waterproof feathers, which enable them
to swim and dive beneath the surface of the water. Ducks create this waterproofing using the oil from
the preen gland, near the base of the tail, which they spread through their feathers and the underlying
dense layer of down with their bills. A layer of fat under their breast skin also helps to keep them
buoyant. Different ducks feed in different ways, some dive, some upend and feed from the bottom of
the lake or river while others ‘dabbles’, stirring up the water with their feet to find food.

Dinosaurs lived for an enormously long time-some 150 million years – before they died out about
64 million years ago. All the dinosaurs disappeared at about the same time. However, whether
this happened over a few days, a few years, one or two centuries, or even a few thousand years is
impossible to say. Many people believe that the dinosaurs became extinct as a result of climate change
after a huge meteor or a small asteroid struck the Earth. The extinction of the dinosaurs was not an
isolated event. At the same time most marine reptiles and pterosaurs also died out.

Several families of birds can be trained to ‘talk’, but in reality they are simply mimicking noises without
really understanding what they are saying, although they can be taught to associate a given word or
phrase with a given action, for example saying ‘give us a peanut’ to get a reward. The best talkers are
parrots and mynas, and members of the crow family such as crows, jackdaws and ravens can also learn a
few simple words. They can do this because the are highly intelligent birds and they are excellent mimics
in the wild. Other bird mimics that do not pick up speech are starlings and some of the bowerbirds in
Australia, including on individual that had learned to copy the noise of the logging machines that were
destroying his habitat.

Construction of the famous white marble bell tower (campanile) of Pisa’s cathedral began in 1173 but by
the time three stories were built, it had already begun to lean. Over the centuries, many methods have
been tried to prevent the lean getting worse but as most people did not understand what was causing
the lean, they made it worse. The soil under the tower is very soft and waterlogged in parts, which is the
most plausible explanation for the lean. In 1990, the tower was closed to the public because cracks in
the walls were worsening and it was feared that it might topple over. Since then, straps and cables have
been used to hold it in place, 800 tonnes of lead weights have been added to the high side of the base
and 38 cubic metres of soil removed from that area, which has stabilized the tower, and even reduced
the lean slightly.

The simplest way to describe an optical illusion is that it is a ‘trick’ that our eyes play on us. We seem to
see something that isn’t really so. Or we may be able to see the same object in two completely different
ways. If our eyes are working properly, and they are instructions for seeing exactly what is before
us, how can they play such tricks on us? This is because vision is not a physical process. It is not like
photography, for instance, which work mechanically. Vision is really a psychological experience, because
it is not the eyes that see, but our brain.

We all dream at night, although we may not remember our dreams in the morning. Although scientists
do not precisely understand how we dream, they have discovered that it is important for us to do so
and people who are prevented from dreaming soon begin to feel unhappy. We are deeply asleep when
we dream, but our brains are active. Scientists think that our brains may be working though the events
of the day, storing memories, making sense of our emotions and preparing us for the next day, Dreams
are often unrealistic or even weird, putting us in odd situations. Bad dreams, called nightmares, may be
particularly vivid and we may be more likely to remember flashes from them because the emotions the
created were so strong that they made us wake up.