Compound Interest

Compound interest is the result of reinvesting or rolling your earnings over so that your reinvested earnings start to earn you even more.  

It's like when you're building a snow man.  That first snow ball is tough, but once you get it rolling downhill, that initial small snow ball can get big with much less effort.  

Compound Interest was much more powerful when interest rates were higher, but it's still the 8th wonder of the World.