Ask Mr Brain...all will be explained

Why is snot green? WONG YUEN-LAM Pui Ying College A few readers have asked that one, along with 'what is it?' The inside of your nose is lined with a protective mucus membrane, similar to that in your lungs and stomach. This lining is the body's first line of defence against invading bacteria, viruses and other unwelcome visitors.

Mucus contains a very useful enzyme called lysozyme which breaks down the cell walls of many bacteria, killing them before they invade the body and cause mischief.

The nose's mucus membrane is covered with tiny hairs which help to trap dust and bacteria and stop them being taken down into the lungs. Because the membrane is permeable, there are white blood cells to defend it against bacteria. When white blood cells die, they are washed out in the mucus. Snot is basically a mixture of mucus, anything the hairs have filtered out and dead white blood cells.

Pus in infected wounds is green due to the white blood cells gathering to fight and kill invading bacteria and viruses. Snot owes its colouring to the same source - heroic white blood cells protecting us by killing hostile bacteria.

What is a black hole and how do astronomers find them? A black hole is a region in space which results from the collapse of a massive star when it runs out of nuclear energy.

Each planet and star has its own escape velocity, that is the speed at which you have to travel if you want to escape from its gravitational field. The larger and denser the planet or star, the higher the escape velocity. Earth's escape velocity is 11.2 kilo metres per second and that of the moon is 2.4 kilometres per second.