Why is there sand on beach and what are sea shells?

Why there is sand on beach?
I have been to more than fifty beaches and one common thing in every beach is sand, but I never wondered why there is only sand on beach and not any other material. During my last trip to Sri Lanka, I have spend more than fifteen days hitting the beaches on the southern and western coasts. On one fine afternoon, while I was sitting on the Marakolliya Beach, in Tangalla district of Sri Lanka this question struck my mind. Then I asked this question to few people on the beach and no one knew the answers. At last an old German was able to answer the question.

The reason why there is sand on beach is because of erosion of earth. During the rainy season the water flowing from the rivers erodes the land (rock and other materials) and makes them into small particles which form sand. The flowing rivers then carries sand with them which finally gets combined into the sea or the oceans. Thus, there is sand on beach. Overall sand is nothing but smaller pieces of rock that gets eroded and eroded over time.

What are sea shells?
Another question that struck my mind is what are sea shells. Are they particular form of rock or are they any living species or are they complex substance formed on beaches due to trash on the beach. Well, infact sea shells are the outer protective layers of few living species of animals that live in water. After these animal dies, the soft part of the body is eaten away by the other animals leaving the outer shell behind. Then due to waves in the sea these outer sells are carried on to the shores.