Earthstar is a specification and Javascript library for building online tools you can truly call your own. By being light, simple, and cheap, bicycles confer huge amounts agency to their users. 'Web3' doesn't have a lot of bicycles. Instead it's got a lot of hyperloops: slick, complex, expensive, and with very little application. What can we learn from bicycles when we design new tools for the web? Earthstar occupies a relatively unexplored band of the contemporary web3 spectrum: ephemerality, lightness, offline-ness, malleability, and serving groups of people who already know each other. This project was funded through the NGI Assure Fund, a fund established by NLnet with financial support from the European Commission's Next Generation Internet programme, under the aegis of DG Communications Networks, Content and Technology under grant agreement No 957073. Applications are still open, you can apply today.