Economics
Public Goods and the Free Rider Problem
Quick fact
Economists define public goods by two traits: non-excludability (you can't stop non-payers) and non-rivalry (one person's use doesn't reduce it for others). Classic examples include streetlights and national defense.
Why this is interesting
Ever enjoyed a public park, clean air, or national defense without paying directly? Why do we rely on governments for these, and why can't the market just provide them?