Mage Knight: Machination
Prerequisites: Knowledge (Architecture & Engineering): 4 ranks, must know at least one Tinkering cantrip higher than 0-level
Benefits: You may “lose” one prepared cantrip of the same level or higher than a Tinkering cantrip (including Tinker) in order to cast that Tinkering cantrip without preparing ahead of time. This feat only applies to versions of Tinkering you are able to prepare (in other words, if Light Tinkering was one of your known cantrips but Moderate Tinkering was not, you could spontaneously cast Light Tinkering but would be unable to spontaneously cast Moderate Tinkering).
Normal: A magus must prepare all of her cantrips ahead of time.