Rare frogs.
Hatched, not bought.

… cards in … epochs of …. The price starts at … and goes up … each epoch.

Said plainly: keeping an egg warm is proof of work. Everyone works on the same puzzle: find a number (a nonce) that makes keccak256(challenge, your address, nonce) lower than the target. Every hatch changes the puzzle, so everyone starts over. There is at most one card per block. Your address is part of the hash, so nobody can steal your find. If someone is first you pay only gas, never the price.

The puzzle aims for one card every … seconds. It adjusts every … cards, at most 4× harder or 2× easier. Each recent hatch doubles the work for a while. Every … quiet minutes halve it. The easiest level gets twice as hard each epoch.

The egg and the next block. A new card has no tier and no frog yet. Its design is picked with the hash of the next block, which does not exist when the transaction is sent. Robinhood Chain makes about ten blocks a second, so this page shows the frog about 0.2 s after the hatch. The next hatch writes it on chain; when the pond is quiet, anyone can do that with reveal(). Until then wallets and marketplaces show the card as an egg. The chain keeps block hashes for about 11 hours, so a frog let out within that time still comes from the block right after its hatch.

Any tool is allowed. This page hatches on your CPU, or on your GPU in browsers with WebGPU (Chrome and Edge on desktop, recent Safari). The puzzle is public, so a hatcher you write yourself works just like this page. The contract calls things by their plain names:

// 1. read the puzzle (state(yourAddress) returns all of it in one call)
challenge = challenge()            // bytes32, changes with every hatch
target    = effectiveTarget()      // uint256, moves with recent hatches and quiet time
price     = currentPrice()         // wei, 0 when sold out

// 2. search
digest = keccak256(abi.encodePacked(challenge, yourAddress, uint256 nonce))
//       32 bytes    +   20 bytes    +   32 bytes = 84 bytes, one keccak block
found when uint256(digest) < target

// 3. send the find from yourAddress (it only works for that address)
mine(nonce)  with value >= price   // extra ETH is refunded
// egg #… also opens the pool: gas limit >= GATE_GAS (…), send ~…

• One card per block. Every hatch changes the challenge, so a find only works until the next one. Check state(you).challenge right before you send.
• A late find reverts with InvalidSolution or OneMintPerBlock. You lose only gas, never the price.
• The target moves. It adjusts every … cards to aim for one card per … s. Each recent hatch doubles the work for a while (one step cools off per … s, … steps at most). Every … quiet minutes halve the work. The easiest level gets twice as hard each epoch.
• Handy views: digestOf(address, nonce) checks a find, currentStreak(), currentBaseTarget(), epochFloor(id). Errors: NotStarted, SoldOut, OneMintPerBlock, Underpaid(price), InvalidSolution, GateNeedsGas(gas).

Contract: not deployed yet on Robinhood Chain. RPC .