Ask Mr Brain..all will be explained

Which animal lives the longest? Some insects live for only a few days, but moving up the evolutionary ladder, living up to 68 years, the eagle owl is among the longest-lived birds. Living on average a few years more, humans and elephants are the longest-lived mammals, with lifespans of about 77 years.

However, while even the oldest humans rarely make it much past 100, the giant tortoise of the Galapagos Islands has been known to live up to 150.

Outside the animal kingdom, the plant kingdom has a lot of members which have very long lifespans. The olive tree can continue happily for more than 3,000 years and the giant sequoia can make it to about 4,000 years. But even these venerable pensioners are youngsters compared to the bristlecone pine, one of which is the world's oldest tree at 4,900 years.

But single-celled organisms are the real immortals. Single-celled organisms do not grow old and die. They reproduce by splitting into two 'daughter' cells and these daughter cells are as effectively as old as their 'parent'. So some single-celled organisms and bacteria are probably billions of years old and still going strong. Just imagine the tales they could tell of what they have seen in their lifetime - the movement of the continents, the ice ages, the rise and fall of the dinosaurs, the evolution of an ape which walked upright and the whole of human civilisation to date. It is just a shame they never evolved enough to appreciate it all.

When you are born, are your adult teeth and baby teeth set in your gums? Your teeth begin as groups of cells called buds. These buds begin to form months before a baby is born. Even the buds for your adult, or permanent, teeth start growing before you are born.

The buds, though, are not really teeth. As they grow, cells on the outside turn into the hard crown that makes the outside of our teeth. Then other bud cells make the roots and insides. Each tooth is still growing as it pushes up through the gum. A baby's first tooth comes in when he is about six months old. The crowns of some baby teeth do form before birth. But the crowns of adult teeth form after birth. Most crowns will form in the jaw by age four.