Before typing, decide which words or phrases are essential to your search and which are non-essential. Essential words are considered key concepts.
Our thesis statement is:
The proliferation of FAST FOOD has led to the national problem of OBESITY.
There are only two concepts in this statement that are essential to its meaning.
We eliminated the word "proliferation" because it modifies the essential concept of fast food.
The phrase "national problem" is not crucial because we assume any article talking about "fast food" and "obesity" will discuss some negative aspect that would represent a national problem.