Samantha Fox, of Ardgowan School, asks :-

What is hen's egg shell made out of?

Abby Smith, a marine geologist who specialises in shell material at Otago University's Department of Marine Science, responded.

All bird's eggs have a hard shell made of calcium carbonate (CaCO3). Calcium carbonate is the same kind of material used to make the shells of snails, clams, crabs and corals - it is also called "lime." It is quite soluble in acid - if you put a hen's egg in vinegar or lemon juice, after a few days the hard part will dissolve away, leaving just the membranes that keep the fluids in.

An egg is a bit like a spaceship - it has to contain everything its resident needs for the journey. There are food and water stores, a living space, a gas exchanger and waste disposal facility. The protective "outer hull" of this spaceship is the shell. The shell protects the embryo against physical harm, temperature changes, and drying out.

Fish eggs and frog's eggs are little and protected by a jelly-like goo. Early land-dwelling reptiles probably had to go back to the water to lay similar eggs. In the Late Permian, over 260 million years ago, some reptiles developed the amniotic egg - one covered by a tough membrane and a leathery or calcareous shell. The membrane and shell are semi-permeable, which means that gases can pass through, but fluids cannot. The growing embryo in the egg can breathe: getting oxygen in and waste carbon dioxide out. It also means that the embryo can drown if the egg goes underwater for too long! The development of the amniotic egg allowed reptiles to be fully land-dwelling in a way that fishes and amphibians could not. The shelled egg is one of the pieces of evidence that birds are the living descendants of the dinosaurs.