Someone in a Facebook group I'm in was looking for an explanation if #carbonDating that would make sense to a five-year-old.
It's tricky because we then have to assume none of the background knowledge most people pick up later! I might want to use pictures, really.
I thought I'd try anyway though...
There are two kinds of carbon. One of them (C-14) is radioactive, which means that it turns into something else over time: we say it 'decays'.
Half of that kind decays every 6000 years (actually 5730, but whatever). So after 6000 years, half of it will be left. After 12,000 years, a quarter. After 18,000 years, an eighth will be left.
So we can tell how old something is by how much C-14 it has! If it has one eighth the amount that living things have, it must be about 18,000 years since it died.
It only really works for things that were alive, because while they're alive they're always taking in new carbon by eating and so on. The amount of C-14 in the air is always about the same.
'But Fergus! Why is the amount in the air always the same? Doesn't that decay too?'
Yes! It stays the same because cosmic rays keep making new C-14, but that's a story for another day...