Full Idea
Lewis pointed out that many-membered classes are nothing more than the mereological wholes of the classes formed by taking the singleton of each member.
Gist of Idea
Sets are mereological sums of the singletons of their members
Source
report of David Lewis (Parts of Classes [1991]) by David M. Armstrong - Truth and Truthmakers 09.4
Book Reference
Armstrong,D.M.: 'Truth and Truthmakers' [CUP 2004], p.118
A Reaction
You can't combine members to make the class, because the whole and the parts are of different type, but here the parts and whole are both sets, so they combine like waterdrops.