Full Idea
The four wheels of a car are parts of it (each is part of it), but there is not a fifth part consisting of the four wheels.
Gist of Idea
Each wheel is part of a car, but the four wheels are not a further part
Source
Peter Simons (Parts [1987], 4.6)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.160
A Reaction
This raises questions about the transitivity of parthood. If there are parts of parts of wholes, the basic parts are OK, and the whole is OK, but how can there also be an intermediate part? Try counting the parts of this whole!