You discover that if you have a bunch of isoduck eggs, half of them will hatch into isoduck chicks every hour.
So if you put 16 eggs in a big box, after an hour, there would be 8 eggs and 8 chicks. And after another hour there would be only 4 eggs and 12 chicks. And so on.
One day one of your farmhands shows you a box. Inside it are 3 eggs and 9 chicks. He tells you that he only put eggs into the box, but now there are both eggs and chicks. You tell him, "I know when you put the eggs in the box."
He asks you how you know that. So kids, how do you know?
That's right! You know that half of the eggs hatch every hour. If there are 3 eggs now, that means an hour ago there were six eggs, and that means instead of 9 chicks, there were only 6 chicks and hour ago. And with six eggs and six chicks, obviously one more hour ago, there were 12 eggs. So your farmhand put the eggs into the box two hours ago.
By counting the number of eggs and chicks, you can calculate how long ago the eggs were put in a box.
Now the isoduck is made up. But in real life there are certain atoms that are radioactive. These radioactive atoms, also called isotopes, don't hatch into ducks, but they hatch into different kinds of atoms. For instance, one kind of potassium isotope hatches into an argon isotope.
And they hatch with the same rules as the duck. Only it isn't half of them every hour. It takes 1.3 billion years for half of them to hatch!
But the rules are the same, by counting the number of potassiums and argons, you can calculate how long ago the isotopes were put in a box.
Of course, there aren't any potassium farmers that put potassium isotopes in a box. But there are certain kinds of rocks that have potassium in them. And argon is a gas, so it shouldn't be found in a rock like this. But when we look, we find there are little argon isotopes trapped inside the rock! When the rock was hot and molten, the argon would all bubble away like steam escaping. So when the rock cools and gets hard, there's no argon inside. So all the argon that we see today came from the potassium isotopes hatching.
By counting the number of potassium and argon isotopes, you can calculate how long ago the rock cooled down and became solid.
And when we do that, we find that the oldest rocks on earth we find are more than 4 billion years old.