Why is beef red and pork white? Meat from chickens and pigs is usually white, while that from cows is red. The difference in colour is due to the amount of myoglobin in the meat. Myoglobin is a protein that carries oxygen to the muscle fibres, in much the same way that haemoglobin transports oxygen in the blood. Animals which are active, such as cows and horses, have higher levels of myoglobin to keep their muscles supplied with oxygen. More sedentary, or lazy, animals such as domestic pigs have little need for high levels of myoglobin and so their muscles - which form their meat - are white. While the meat of lazy domestic pigs is white, their wild cousins - boars - lead an active life in the forests of Europe and consequently their meat is red. Similarly, ducks, which are more active than chickens, have red meat. Is the Great Wall of China really visible from the moon? After being told how long the Great Wall is, people are usually told that it is the only man-made object on Earth visible from the moon or space. While the Chinese tourist industry might not like it, the truth is that - given the limited resolving power of the human eye - it is not visible from the moon, nor even from the edge of space. While the Great Wall is more than 2,400 kilometres long, it is only about six metres high and six metres wide, in the places where it is still standing or has been renovated. With a width of only six metres, the Great Wall would only be visible with the naked human eye from an altitude of about 20 kilometres, which is far short of the 384,000 kilometres the moon is distant from Earth. Even with its shadow, it is thought to be visible from about 60 kilometres at most, which is still well within the Earth's atmosphere. To set the matter to rest, astronaut Neil Armstrong said the Great Wall was not visible from the moon, and he is one of the very few people who have actually been to the moon, so he is in a position to know. Fellow astronauts Jim Lovell and Jim Irwin also said the structure was not visible. While the Great Wall is not visible from space, other man-made things are, such as reclaimed land in the Netherlands (and, presumably, Hong Kong), cities at night (because of their bright lights) and concentrations of oil wells (due to their flares). How does the human body digest food? As soon as you put food into your mouth and begin to chew it, you are helping it break down. Saliva in your mouth has an enzyme, ptyalin, that begins this work. Pepsin is an enzyme in the stomach that, together with hydrochloric acid, helps to break down food. When the food is churned up and partially broken down, it goes to the small intestine where even more enzymes attack it. Some foods such as peanuts, chocolate, milk, bananas, wheat and citrus fruits can trigger allergic reactions in some people such as shortness of breath, pimples or stomachaches.