Why some creatures born from an egg, and others not.

What's the difference between creatures born from an egg and others?

That depends on the history of evolution that each creatures trailed.

Evolution means that the structure of body changes along the change of environment.

The first creature on earth was born in water.

That was the very simple creature like that divide into two when they grow large.

That has evolved to creatures thas have complex structure over very long time.

First creature's eggs don't have shells, because they live in water, so their eggs don't need to be hard.

Eventually creatures that live on the ground have appeared. On the ground eggs need to be hard to avoid to dry up. Furthermore, creatures that give birth to a baby directly have appeared too. that's us, mammals.

Before giving birth, mammal's babies grow up in the womb from soft egg to baby.

The number of babies giving birth once depends on the shape of eggs and babies.

Creatures in water (Fish, flog, etc) lay many soft eggs once, their eggs are often eaten by enemies. after laying eggs, they live apart from babies.

Creatures like birds lay about 3 to 5 hard eggs once. after laying eggs, they live with babies for a while.

Mammals give birth few babies once. after giving birth, they live with children until children will be adult.