Toly’s train pulled into a town with three clock towers. The North Tower said it was Tuesday. The South Tower said it was still Monday, technically. The East Tower had stopped arguing and was ringing just to make a point.

Then Pim flew in. Pim was the town courier, a pigeon in a postman’s cap who numbered every envelope and took the order of the post very seriously.

“Letters get delivered in the order they were sent,” Pim announced, landing on Toly’s suitcase. “The trouble is, nobody in this town can agree when anything was sent.”

Pim told Toly about the town’s great scandal. The baker sent a letter: I’ll buy your flour for ten coins. A moment later the baker’s rival sent one too: I’ll pay twelve for that same flour. The miller got the baker’s letter first. The miller’s cousin, across town, heard about the rival’s letter first. Each of them was sure they knew which came first.

That’s the hard part of running a shared ledger. Order matters. Whoever spends a coin first should win, and nobody should get to spend the same coin twice. But messages travel at different speeds, so two computers can see the same events in a different order.

In 1978 a computer scientist called Leslie Lamport wrote a famous paper about this exact puzzle. Solana’s smallest unit of currency, the lamport, is named after him, which Pim finds deeply satisfying.

Pim set up a test in the square. Two letters, two houses, two different routes. “Watch,” said Pim, “then tell me which one came first.”

Try it. Whichever house you stand in, you’ll be sure of the answer, and someone in the other house will be just as sure of the opposite. Nobody’s lying. What you’re missing is a shared way of saying this happened before that, one anybody can check without having to trust anybody else.

Toly scribbled in his notebook the whole way home. Tick rode on his shoulder, ticking. Somewhere between the second tick and the three-hundredth, the boxes and arrows started to make sense.