Explain What Compound Interest Does to a Small Regular Saving

Everyone says start saving early because of compound interest, and almost nobody explains what that actually means in pounds. This runs one worked example on a monthly amount over several years, splits out how much came from you and how much came from interest, and makes the maths land.

Explain, in plain English and with one worked example, what compound interest actually does to a saving of {MONTHLY_AMOUNT} put away every month at [an interest rate]% over [5] years. Show how much of the final total is money paid in versus interest earned, and make the point land with one comparison a beginner would recognise (like the difference between a jar of coins and a snowball rolling downhill).