Your shoes are dirty

Wordlessly, you cross your legs and lift your foot to the adorable volunteer maidservant's lips. She seems confused for a moment, but then works out what you want and begins to lick and suck at the soiled leather. The dust and dirt of a day's activity disappears under her diligent tongue and once the upper is complete, she turns her attention to the sole.

When she finishes, you change legs and she repeats the demeaning service on your other shoe. By the time this one is as spotless and shiny as its pair, your grad-student has grown quite enthusiastic about her new duties and seems almost disappointed when they are over.