Playground Lessons: Playgrounds are a learning environment. They are an open, public infrastructure with ready obstacles to unleash your creativity. Slips and falls are painful, but the damage is minimized by design. Small scale failures teach large scale lessons and prepare children to navigate physical games with higher stakes. In the blockchain world, \"testnets\" are the playgrounds. Replicas of their parent blockchains, testnets are to operate without economic consequences, lowering the cost of failure and increasing experimentation. The Ethereum community is home to several testnets (here) serving a dual purpose: 1) to test smart contract applications and 2) to simulate changes to the underlying protocol. Ethereum's upcoming transition from proof-of-work to proof-of-stake could be the most important protocol change in blockchain history. It is the engineering equivalent of swapping an engine on a racecar while driving. Trial runs are essential. Ropsten, one of the original Ethereum testnets, was the first to Merge (details here) and will be abandoned after the mainnet makes the same change. Sepolia, a new testnet, also completed the Merge in early July (here) and will take over as a primary venue for smart contract application testing going forward. Each testnet exposed areas of weakness. Developers responded quickly and collaboratively. These small failures have paved the way for a third and final testnet Merge of Goerli, targeted for Aug 6-12th (here). Barring unexpected difficulties, the final step will be to Merge Ethereum mainnet on the target date of Sept 19th (here). All eyes are focused on the main event. If Ethereum is successful, it will be a triumph built on the bumps and bruises of playground lessons. Play on.