Ask Mr Brain..all will be explained
Why is it that Aids cannot be cured? CHAN HIU-GUK Wong Shiu Chi Secondary School Aids is caused by a virus which destroys many of the body's white blood cells. This leaves the body with fewer defences against infection and disease.
Bacteria are relatively large and live outside human cells, which makes it easy to use drugs to kill them without harming the patient.
Viruses are hard to fight because they are tiny and live inside the cells of the person they have attacked. This means you have to develop a drug which could kill what was inside the cell without killing the cell itself.
One of the main areas of research in the fight against Aids is in trying to develop a vaccine against the Aids virus. This is because the body is much better - and safer - at fighting disease-causing intruders than any drug, as long as it knows what to look for. Vaccines work by showing the body what to look for so it can prepare its immune system for a possible attack.
The smallpox virus has been eradicated from the world through the use of a very effective vaccine. Good vaccines are also available for polio and rabies - which are also caused by viruses.
So why isn't there an Aids vaccine yet? The problem is that the Aids virus, like the viruses which cause the common cold, is very cunning and mutates into new forms very quickly, making it hard to develop a good vaccine to defend the body against it.
Each new strain would need a new vaccine, and by then the virus could have changed into another, vaccine-resistant, form. But scientists are still hard at work trying to find a cure.
Dolphins are mammals but why can't they live on land? HUNG WING-FUNG Yuen Long Merchants Association Secondary School Mammals are the highest class of vertebrates.
Mammals are warm- blooded and the females produce milk to feed their young.
Bats, kangaroos, whales, dogs, cats and humans, are mammals. There are 400 or more species of mammal - on land, in the air and in water.
Those that live in water are called aquatic or marine mammals, and dolphins are one of them. Dolphins actually evolved from land-faring creatures millions of years ago and are now perfectly adapted to life in the sea.
On land, the size of mammals is limited by how much weight their limbs can carry, but in the sea their weight is supported by water, which allows them to grow to a much larger size. In addition, larger mammals lose heat more slowly, which is an advantage in cold marine environments.
Dolphins belong to the group of whales called the toothed whales. They have torpedo-shaped bodies without external ears or hind limbs.
Dolphins breathe through a single nostril, or blowhole, on top of their heads.
They are among the most intelligent of animals. They have very large and folded brains - some species have brain-to-body mass ratios equal to that of humans.