[datashovel]

To describe this concept I've always used a boat as the subject.

Imagine you have a boat. Now take a piece off of it. Is it still a boat? How far can you go until it no longer holds the distinction of being a boat?

[mannykannot]

I imagine 'when it sinks' would work for most people.

[datashovel]

So if I understand correctly, then twigs and leaves could be boats? :) I guess if a boat was originally constructed of a bunch of twigs (some of which still had leaves on them), but caught on fire. One lucky leaf was knocked off and managed to survive the fire. Everything else sunk. The single remaining artifact of the boat was that one leaf. And it's still floating... Everyone looked at me perplexed when I told them all that we were looking at a boat! :)

[EdwardCoffin]

And if it is constructed entirely from wood that floats?

[mannykannot]

Then it may still function as a boat. As in all of these examples, X-iness is context-dependent, but for some boats, unlike the other examples here, there actually is something approaching an objective decision procedure.

Yes, I know that submarines are colloquially known as (and are in fact) boats.