Usually one keyword isn't specific enough for your topic.
The boolean operators you need to understand to start out are AND and OR.
Focus your search by adding more search terms and joining them with the word "and".
When you use "and" between terms, you specify that both/all terms must be in every item found.
Using "and" narrows your search. You find fewer items, but they're more specific because they all contain all the key concepts.
The words can be in any order and in any searchable place in the record.
Broaden your search by adding more search terms and joining them with the word "or".
Using "or" broadens your search. You find more items, because they only need to contain some of the key concepts.
Be careful of using both "and" and "or" in the same search request. If you don't put brackets ( ) in the proper spots, the system might search the words in the wrong order, giving inaccurate results.
Try to do all the "or" searches first, then look for limit options to limit your results to those with an "and" term.
If you must use both "and" and "or", place brackets around the "or" terms:
crayfish and (river or stream)
Not all search engines act exactly the same. Although they are all based on boolean logic and understand boolean operators (AND and OR), they might make certain assumptions.
Better yet, look for an "Advanced Search" option, which helps you organize your search terms by laying it all out for you.
Here's Google's advanced search screen:
See the Library Catalogue tutorial for an explanation of the catalogue's advanced search.
When you type two words together, a search engine might assume and AND or OR between them:
If you want to keep them as a phrase, quotes are often a good idea: "great lakes".
Other search engines may have different rules, and you need to check under "Help" to see how to keep a phrase together.