After oxygen, silicon is the most common chemical element found in combination in the rocks that make up the Earth’s crust. Very few people, however, have ever seen pure silicon. That is true not because silicon is invisible, but because it is usually found in combination with other elements.
What does the first sentence tell you about oxygen?
It combines with silicon.
It comes from silicon.
It is the most common element in rocks.
It is important for life on earth.