Cars are supposed to have 4 wheels to drive properly. If one is missing the car can not drive properly. If there is and extra wheel protruding from the middle of the care, it will not work as designed.
A quartet is supposed to have 4 players. They usually have music written for that many musicians. They can still play music, but not correctly.
METAMIA is a free database of analogy and metaphor. Anyone can contribute or search. The subject matter can be anything. Science is popular, but poetry is encouraged. The goal is to integrate our fluid muses with the stark literalism of a relational database. Metamia is like a girdle for your muses, a cognitive girdle.