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 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).
