When is a shoe not on? Figured it out yet?

When is a shoe not on?  Have you figured out the answer yet?  If not, here it is!

When is a shoe not on?  When it's in . . . when it's a "shoe-in" (or rather, as your English teacher would tell you, a "shoo-in"!)                                                                  Above Riddle Copyright MarashGirl©2022            

N.B.  This riddle works fine when it is spoken!  Not so well in print!