Twitter 7 Day Challenge Wrap-up

Remember last Winter a friend had posted a 7-Day Challenge on Facebook.  Well, now another friend has posted a Challenge on Twitter.......Post a B&W image with no explanation.......The series need not be related images, although mine has a common thread.......

DAY 1:

DAY 2:

 DAY 3:

DAY 4:

 DAY 5:

DAY 6:

DAY 7:

Have you Guessed the Common Thread?