Carbon dioxide is unusual because it doesn't go through the usual three phases of matter, from solid to liquid to gas, but it goes straight from solid to gas. The volume of the gas is much greater than the volume of the solid. When a solid turns into a gas, we say it sublimes. The process is sublimation.

I don't know whether it is important to study science at a young age, though current thinking emphasises the need.

In prehistoric times, Homo sapiens was deeply endangered. Early humans were less fleet of foot, with fewer natural weapons and less well-honed senses than all the predators that threatened them. Moreover, they were hampered in their movements by the need to protect their uniquely immature young - juicy meals for any hungry beast.

My own field, the prevention of genetic disorders in babies, has been possible only because of humane work on animals.

Women of child-bearing age steadily run out of eggs by the continuous process of cell death. While reading a copy of the Guardian carefully from cover to cover, a normal woman will have lost on average two eggs - while, typically, a normal man will have made 70,000 new sperm.

Man is a competitive creature, and the seeds of conflict are built deep into our genes. We fought each other on the savannah and only survived against great odds by organising ourselves into groups which would have had a common purpose, giving morale and fortitude.

I did not study science at school until I was 13, when I was totally turned on by a seemingly dreary old teacher who suddenly, unannounced, manufactured a huge explosion in the middle of a totally boring monologue. From then on, all of his class wanted to make explosions.