The fee was forty-seven dollars.

I want to be clear about this, because I'd been told a number of things about DeFi that had turned out to be romantic exaggerations, but I had specifically been promised - by Dev, by articles, by a podcast I'd listened to in the bath - that the future of money was going to be cheap. Banks, I had been told, charged fees because they were extractive dinosaurs. In the bright new world, transactions would cost pennies.

I was trying to send 0.001 ETH to Dev. The network wanted forty-seven dollars - or something like 0.02 ETH - to do it.

"Moolah."

We were back at the bench. It was colder than the previous week, and the sky had that late November flatness that made even the pond look like a photograph. Moolah was wearing, for reasons she had not explained, a tiny knit scarf.

"Yes, Priya."

"Please explain this."

She peered at my phone. "Ah. You picked a bad moment. Try again on Sunday morning."

"That's not an explanation."

"No. It's advice. The explanation is longer." She settled in. "When you send a transaction on Ethereum - or most blockchains - you are paying for a seat. The network can only process so many transactions per block, and blocks only come every few seconds. There is always more demand than supply. So there is always an auction."

"An auction."

"For every block, yes. You and everyone else who wants a transaction processed are silently bidding against each other for a limited number of seats. The highest bidders get in. The rest wait. You are not paying a fee for the transaction itself - you are paying to cut the line."

Dev, who had clearly been waiting for this moment, leaned in. "It's like surge pricing -"

"Dev. Let me."

He deflated.

"Imagine a highway with one toll booth," Moolah went on. "At three in the morning on a Tuesday, you roll through for pocket change. During Friday rush hour, the queue is an hour long, and there's an express lane that costs fifty dollars. The highway itself hasn't changed. Its capacity hasn't changed. But you are competing with ten thousand other drivers, and someone has decided to auction off the fast lane."

I looked at my screen again. Forty-seven dollars. "And right now it's Friday rush hour."

"Right now," Moolah said, "there's a popular new token launching. Ten thousand people are trying to buy it in the same three-minute window. They are all bidding. You are bidding against them, even if you don't know or care about the token."

"So if I wait -"

"If you wait until tonight, or Sunday morning, you'll probably pay a dollar. Maybe less. Same transaction. Same network. Same capacity. Just a quieter moment."

She sipped her latte. A fresh one. Though neither of us had seen her go to the counter.

"Here is another way to see it. Imagine you want to post a letter, but the post office sells only a hundred stamps per morning, and today there are ten thousand people in line. If you want yours in today's mail, you have to out-bid someone else. You buy the stamp from a scalper. The stamp itself is not worth more than yesterday. The urgency is."

I thought about that. "So the fee isn't for the service. It's for the priority."

"It's for both. A little baseline charge that keeps the network from being spammed - that's the true cost of the service. And on top of that, a tip, which goes to whoever processes your block and lets you jump the queue. In the jargon you'll see: base fee plus priority fee. Base is the cost of mailing the letter. Priority is the tip to the courier."

"And it's measured in…"

"Gwei." Moolah said the word without pleasure. "Which is a tiny fraction of an ether. The units are stupid. Don't worry about the units. For most of us," I couldn’t help but give her a slightly confused look, but she carried on, "it’s the dollar number at the bottom of the screen that matters. Always check that number."

Dev stirred his coffee thoughtfully. There was already a ginkgo leaf in it. "Tell her about the hacks, though."

"Oh." Moolah's whiskers twitched. "Yes. The grim comedy of the gas market. If you are a hacker, and you have just stolen a million dollars and need to get it off the network before anyone can stop you, you will pay enormous gas fees to get into the very next block. I once watched a thief pay two thousand dollars in fees to escape with stolen funds. The network does not care that he is a thief. The auction does not care. Highest bidder wins. The fee is the only gatekeeper, and the gatekeeper only understands money."

"That's bleak."

"It's honest. Which is what I keep telling you this system is."

I thought about the highway, the post office, my forty-seven-dollar transaction fee, and the popular new token I didn't know existed, plus the ten thousand strangers silently outbidding me for a three-minute window I hadn't chosen.

I put my phone down. "I'll send it Sunday."

"Good."

On Sunday morning, from my kitchen, I sent twenty dollars to Dev for eighty-three cents.

He sent back an emoji of a small crown.

I wrote BASE FEE + PRIORITY FEE = TOTAL GAS in my notebook, and underneath, in smaller letters: always check the dollar number.