Mathematics
Connectedness: The Topological Idea of Being One Piece
Quick fact
In topology, the interval [0,1] and the circle are both connected, but the circle is not homeomorphic to the interval because removing a point from the interval disconnects it, while removing a point from a circle leaves it connected.
Why this is interesting
Think of a coiled rope and two separate shoelaces. Both look like strings, but one is 'all one piece' and the other is not. How can mathematics capture such a basic difference?