If you write _are_ or _is_ in the first 40 characters of an excerpt, the part before, and including, _are_ or _is_ is removed when showing the excerpt. That is the reason why most excerpt starts with "questions about," "topics related to," or something similar.

[This answer][1] reports the regular expressions used to truncate the excerpt; in short, you cannot start an excerpt with, for example, "this tag," "this tag is," "this tag is used to," the [foo] tag," "the tag [foo]," "blah blah is," or "blah blah are" (replace "blah blah" with any phrase that uses less than 37 characters).


  [1]: https://english.meta.stackexchange.com/questions/886/tag-excerpt-inconsistently-displayed/887#887