Sometimes the Source Editor gets really cluttered with open files. Instead of individually closing the files you don't want, by clicking the little cross on the right side of each file's tab at the top of the Source Editor, you can just right-click the tab for the file that you do
want to keep... and then choose Close Other Documents:
When you do this, all other files are closed and only the one that is currently selected remains open. That saves you a lot of clicking.