Nicotine is the main component of tobacco and is chemically classified as an alkaloid.

Alkaloids are a type of compound produced by plants.

This reaction occurs through a chemical process.

In this library, books are classified by genre.

He categorized the books by genre.

He quit smoking tobacco.

Smoking cigarettes is not good for your health.

He is majoring in chemistry at university.

He has a habit of smoking tobacco every day.

He was reading the newspaper while holding a cigar in his mouth.