We explore the idea of adding bidirectionality to spreadsheet formulas, so that editing the output can directly affect the input. We introduce portals: a portal is a value paired with its where-provenance, that is, one or more links to its origin. When a portal is the result of a formula in a cell, that cell inherits the capability to edit the locations described by the provenance of the portal. The simplicity of portals makes them amenable to implementation in an existing spreadsheet system. We analyze the provided functions list of large spreadsheet system and find that many frequently used functions work with portals for free.