I had a friend recently ask me about the use of the particle "the" in different contexts that she was having trouble understanding and I found I was unable to explain the use. The lesson she had done was similar to this one:

<http://www.myenglishpages.com/site_php_files/grammar-lesson-articles.php>

Where "the" is explained as a word for a specific noun known to both speaker and the listener such as 

> 
> The bike was red.
> 
> 
> 

However a number of the examples she was given(and were not really explained in the material she showed me) were of the form

> 
> The bike is one of the most efficient means of transportation.
> 
> 
> 

or

> 
> The squirrel is a small mammal
> 
> 
> 

Neither of these examples are referring to a specific noun, they are referring to a generalization, "The bike" is true for every bike and the same for the squirrel example. Further to her confusion was that the sentences were not plural when referring to a generalization of "every bike" when she tried to correct the sentences as "the bikes are". 

Would it be possible for someone to give me a explanation of this use or point me to something that has an explanation. I found it quite difficult to try and explain and the best I could come up with was that it's similar to "The idea of a bike is. . ." or "The idea of a squirrel is. . ." which I don't think is entirely accurate. The main trouble comes from "The" being used for a specific item, and in these cases the noun in question being used as a generalization. Am I just thinking about this wrong?