Economy Talk

How would you explain inflation to a 10-year-old?

My niece asked me why her favourite chocolate bar costs more than it did "when she was little" (she's ten, so this was about three years ago, ha!).

I'm a researcher and I completely froze! I started saying something about money supply and she just stared at me.

So: what's your best simple explanation? Analogies welcome! I want something she'll remember, not a lecture!