The Five-Year-Old's Guide to the Galaxy

ELI5 mod flabbergasted1 made a list of some of the best ELI5 responses. Source.

Below is a hand-picked collection of outstanding explanations from this subreddit. Each linked answer is not only informative and correct, but written in terms that an elementary school student would legitimately understand. If you find an equally exceptional explanation not on this list, make a base-level comment on this thread and it will be considered for addition. Read and enjoy!

Economics

 * Debt in a Money-Based Economy by Hapax_Legoman
 * Expansionary Monetary Policy by GOD_Over_Djinn
 * Libertarianism by AmazingSyco
 * Stocks and the Stock Market by CarlH
 * Trust Funds by The_Cleric

History

 * JFK Assassination by Didji
 * World War I by Axon350

Literature and the Arts

 * The Catcher in the Rye by TrouserDemon
 * Baroque vs. Classical vs. Romantic Music by HellOnTheReddit

Mathematics and Logic

 * Anything to the Zero is One by LordAurora
 * Bases by Didji
 * Chaos Theory by Captain_Kittenface
 * Crash Course in Logic by gmanp
 * Manifolds and the Poincaré Conjecture by flabbergasted1
 * Negative Times Negative Equals Positive by lampochka_returns
 * Occam's Razor by OtherSideReflections
 * P versus NP by flabbergasted1
 * Riemann Hypothesis by flabbergasted1

Philosophy & Religion

 * Existentialism and Nihilism by Semiel
 * Islam by meowtiger
 * Nietzsche by plaidpant
 * The Qur'an by dottxt

Recent Events

 * London Riots (August 2011) by chetney
 * Phone Hacking Scandal (August 2011) by Didji
 * The US Drops from AAA to AA+ (August 2011) by uriman
 * What If Greece Defaults (October 2011) by duckymf
 * SOPA (November 2011) by flabbergasted1

Reddit

 * The Front Page by flabbergasted1
 * Vote Fuzzing by kissmyapp

Science

 * Domesticating Animals by josh6499
 * Fire by Balestar
 * The Nervous System by Scriptorius
 * Space-Time by 4x4prints
 * The Speed of Light by Avedomni
 * Plasma by wiz3n

Technology

 * Buffer Overflow by UnitedStatesSenate
 * Cell Phones by The_Cleric
 * Electronic Ink by GSnow
 * Hashing by AndreasTPC
 * HTTP by The_Cleric
 * Internet by EdgeOfDreams
 * ISPs by Didji
 * .JPEG vs. .PNG by asokoloski
 * LCD vs. LED vs. Plasma by unndunn
 * Linux vs. Windows vs. OS X by TickTak
 * Net Neutrality by Didji
 * Programming Languages by chipbuddy

U.S. Politics

 * The Debt Ceiling by The_Cleric
 * Liberalism vs. Conservatism by Didji
 * "Obamacare" by Didji

World Politics

 * Africa by bkoatz
 * Fascism by blackstar9000
 * The Israel-Palestine Conflict by nathanite
 * North Korea by elloelloello
 * Wikileaks by Devistator