Back in the attic, Toly gave Tick a job. On every tick, Tick would take the fingerprint of the tick before, mix in a count and stamp out a brand-new fingerprint. Tick loved it. It had been waiting its whole life for a job that was nothing but ticking.

Before long a trail of tiny stamps ran out of the attic and across the meadow, each one depending on the last. You couldn’t jump to stamp five hundred without making the first four hundred and ninety-nine. Without quite meaning to, Tick had made a record that proves time has passed.

The fingerprint is called a hash, a short code worked out from some data. Change one comma in the data and the hash comes out completely different. Proof of History runs a common hash function, SHA-256, over and over, and each new hash is made from the one before. That’s why the chain has to be built in order, one step at a time.

When a message arrives, its fingerprint gets mixed into the chain, which pins it in place: after this hash and before that one. Anyone can check the trail afterwards, and checking is quicker than making it, because different stretches can be checked at the same time.

“So it’s a clock,” said Pim, who had flown in to inspect. “It shows what order things happened in,” said Toly. “Agreeing on which trail is the real one is a different job, for a different meeting.” Pim made a note. Pim always makes a note.

Pim turned up with a sack of letters that had got muddled on the way. Each one had been stamped into Tick’s trail, though, so the trail still remembers the right order.

Your turn, reader. Put the letters in order and press Check the trail. The book works out real SHA-256 hashes in your browser, chains each one to the hash before it and compares your chain with Tick’s. Swap two letters and watch every fingerprint after them change.

Pim checked the result twice, then once more for luck. “Order: settled,” Pim announced. “Agreement: pending.” That night a note slid under the attic door, sealed with an owl-shaped stamp. Midnight. Bring the clock.