Finding the Main Idea

The earth's surface is constantly changing.  All over the world everyday there are earthquakes, floods and volcanoes that quickly change our world. Some changes are too small to see.  Others cause tremendous damage and costs millions of dollars to clean up.
What is the main idea?

Have you ever wondered where chalk comes from?  Tiny animals called foraminifera live in the sea.  Each of these small living animals makes a tiny shell out of lime.  When they die, their shells remain.  With time, these shells become layers of white rock which we call chalk.  Chalk is often found in places that were once underwater.
What is the main idea?

Keep your teeth happy. Brush your teeth twice a day. Make sure you floss your teeth every time you brush and after you eat. Avoid eating anything that sticks to your teeth, like candy and syrup. Eat vegetables and fruits that don't cause cavities. 
What is the main idea?

Sharks can lose up to 30,000 teeth in their lifetime. Sharks have teeth arranged in rows inside their mouths. These teeth do not have roots like human teeth do, so they fall out easily. When a shark takes a powerful bite into its food, several teeth are lost. When these teeth are lost, new ones push forward from the back rows to replace them. Luckily, sharks continually grow new teeth to replace the thousands they lose.

What is the main idea of the following paragraph?
Mother gorillas do a lot for their babies. They nurse their babies. They protect them from danger. And they teach the tiny gorillas how to get along with other gorillas.

What is one supporting details from the text?
Mother gorillas do a lot for their babies. They nurse their babies. They protect them from danger. And they teach the tiny gorillas how to get along with other gorillas.