Transparent Aluminum

Scotty and Dr. McCoy have revealed the chemical formula for a 23rd-century material to a late 20th-century manufacturer. They take a brief moment apart to discuss the implications of this impending temporal paradox. 

You realize, of course, if we give him the formula, we're altering the future.
 - Why? How do we know he didn't invent the thing?
[Bones double-takes, his mind rapidly considering the sound satisfaction of this logic.]
Yeah...
[They exchange knowing smiles and proceed.]