A hook is an opening statement which is usually the first sentence in the introductory paragraph that attempts to grab the readers attention. There are several types of hooks which are definition, proverb or idioms, quotes , interesting facts,  and so on. 

1) The negative impacts of social media on teenagers. 

2) Should more employers offer flexible scheduling or working from home? 

3) Interracial marriage 

4) The benefits of learning a foreign language at school. 

5) Vaccination