A speck smaller than a grain of sand sits in a rock in Western Australia. Its first home has worn away. The speck has outlasted it by billions of years.
You are looking at zircon, a tough crystal from the Jack Hills. Some grains are about 4.4 billion years old. Heat and wear destroyed the rocks they grew in, but the loose grains washed into younger stone, like pebbles pressed into wet cement.
Zircon keeps a clock inside. As it grows, it takes in uranium and shuts most lead out. Uranium slowly turns into lead. Count the atoms and you can work out when the clock began.
Then comes a surprise. Oxygen in the crystal carries a clue to water. Its mix of heavy and light atoms suggests that the rock which later melted to make the zircon had touched liquid water near Earth's surface.
That pokes a hole in the picture of a young Earth covered in endless melted rock. Some crust cooled enough to get wet much earlier. How much of Earth looked that way? Nobody knows yet. The clock gives the crystal's age; the oxygen hints at the water. Both clues fit inside that grain.
