I think that I shall never see
A graph more lovely than a tree
A tree whose crucial property
Is loop-free connectivity
A tree which must be sure to span
So packets can reach every LAN
First the Root must be selected
By ID it is elected
Least cost paths from Root are traced
In the tree these paths are placed
A mesh is made by folks like me
Then bridges find a spanning tree
-- Radia Perlman.
Extract from her paper on the Spanning Tree Protocol.