Mathematics
Trees, Spanning Trees, and the Minimum Spanning Tree Problem
Quick fact
Any connected graph with n vertices has at least one spanning tree, and every spanning tree has exactly n-1 edges. The minimum spanning tree problem can be solved in near-linear time with sophisticated algorithms, and the greedy approach always works.
Why this is interesting
Imagine you need to connect every computer in a building with the least amount of cable, avoiding loops. How would you guarantee the cheapest network?