It's a known fact that it's easier to clean things when they're wet. To do so, people will usually use soapy water or a chemical cleaning agent. However, soap, cleaning agents, or even water might not be on hand to use in a situation where something must be shined up. That, or the character just doesn't find reaching for those things necessary, knowing that they always have a liquid conveniently at hand to use — their own saliva.
This trope might manifest in a character using their saliva to scrub a table or counter until it shines, to quickly style or smooth their eyebrows (or the eyebrows of someone else, especially if that other person happens to be their child), or to wipe unsightly dirt off someone's face. Doing this may be seen as pretty squicky to some, but it can still get the job done all the same.
- In Eskimo Day, Simon spits on his elderly father's suit to clean off some dirt, which causes some indignation.
James: I wondered where I'd draw the line! Being spat on.
- Some servants do it with the royal crown in The Madness of King George.
- Space Jam: The Looney Tunes gang cleans their gym this way.
- Ragetti does this with his wooden eye a couple times in Pirates of the Caribbean. Later on, Barbossa gives the thing a really good slurp and shoves it back into Ragetti's empty socket. The poor man looks terrified.
- Blaze and King Charles II have a bonding moment spitting to shine their leather boots in the French comedy Delusions of Grandeur.
- In A Field in England, Friend does this to Whitehead's hand after getting dirt on him.
- In The Ice Pirates, due to water being in short supply (water/ice is used as a commodity), the shoeshiner says no more spit for the shine until more water is released to the public.
- Jayne of Firefly does this with a knife, combining this with Licking the Blade. This...disconcerts the rest of the crew.
- The Big Bang Theory: Raj uses his spit to clean off dirt from the crotch area of an Aquaman action figure bought in a garage sale.
- On an episode of Seinfeld, where George is forced to be a butler for his parents, Frank orders George to shine his shoes, and George spits on the shoe once Frank's back is turned.
- The bartender in Monkey Island 2: LeChuck's Revenge used his spit to clean the glasses.
- Done by Hank when he must shine his father's shoes as punishment on an episode of The Venture Brothers.