Compound Interest Explained Simply

You know compound interest matters and you still couldn't explain it at a dinner party. This lays it out with one starting amount tracked across ten, twenty and thirty years, so the cost of starting late stops being a vague worry and becomes a number you can see. Ends with the one thing to do this week.

Explain compound interest to me as if I am intelligent but have never studied finance. Show me a concrete numerical example over 10, 20, and 30 years with the same starting amount. Make the difference between saving early and saving late obvious from the numbers. Then give me the one practical thing I should do this week if I am not already taking advantage of it.