Thinking of that thing which an animal body is built around, porous but rigid, consisting mostly of calcium compounds, and usually containing (solid and liquid portions of) marrow, but in this raptor's wing support strut, just a gas, air:
(from a book by John Z. Young, drawn from a photograph in D'Arcy Thompson's "On Growth and Form")
Mastodon is a "FOSS" social sharing hub. A multi-host substitution for capitalistic platforms, it avoids risking a particular company monopolizing your communication. Pick a host that you trust — you can still talk with all hosts running Mastadon. Any individual can run a Mastodon instantiation and join in this social hub in a jiffy.