Why do animals have so many breasts while humans have just two? SILLY PIG Most animals do not have any breasts. Breasts, or mammary glands, are restricted to mammals, which is where we get the name 'mammal' from. Reptiles, amphibians, fish and birds do not have breasts. Mammals can have between one and 11 pairs of mammary glands. Generally, mammals which have large litters, such as pigs and dogs, have more mammary glands so they can feed all their young. Humans and many other mammals which tend to have just one offspring at a time have to make do with just two. Is size of brain an indication of intelligence? The short answer is no. In humans, the adult brain weighs about 1-1.5 kilograms. An elephant's brain weighs about six kilograms and a whale's weighs more than 10 kilograms. Despite possessing such large brains, neither animal can tie its own shoelaces, let alone explore space or program a VCR. Humans, with their much smaller brains, are capable of doing all these things and more. What does count, to some extent, is the ratio of brain size to body weight. The brain of a human is a much larger proportion of his body mass than that of other animals. An elephant's brain may be four times larger than a human's, but an elephant can weigh up to 100 times as much as a human. If we compare like with like, Neanderthal man (who lived about 100,000-40,000 years ago) had a brain larger than modern man, yet was not very intelligent. Why do trees provide oxygen when there is already oxygen in the air? JENNIFER CHEUNG Kowloon Junior School If it wasn't for trees and other plant- life, there wouldn't be any oxygen in the atmosphere. Billions of years ago, there was very little oxygen in the atmosphere. In this primordial atmosphere, blue-green algae were just about the only life forms. Fossils of these simple single- celled organisms have been discovered in rocks 1.5 billion years old, and their descendants are still around today. Scientists say blue-green algae are half-algae, half-bacteria. At some point in the dim and distant past, these blue-green algae started photosynthesising. Photosynthesis is the process used by plants, some algae and a few other odd life forms, by which they use water and light energy from the sun to produce chemical energy for themselves. A by-product of photosynthesis is oxygen. So, for millions of years, these blue-green algae produced oxygen from water and sunlight. When the oxygen level was high enough, other life forms were able to evolve and later the job of supplying oxygen was taken over by trees, which are better at it than blue- green algae.