Full Idea
Composition as identity implies that no persisting object ever changes its parts, which is clearly false, so composition as identity is false.
Clarification
'Composition as identity' (Lewis) says a thing just is its parts
Gist of Idea
Composition as identity is false, as it implies that things never change their parts
Source
Trenton Merricks (Objects and Persons [2003], §1.IV)
Book Reference
Merricks,Trenton: 'Objects and Persons' [OUP 2003], p.22
A Reaction
Presumably Lewis can say that when a thing subtly changes its parts, it really does lose its strict identity, but becomes another 'time-slice' or close 'counterpart' of the original object. This is a coherent view, but I disagree. I'm a believer.