This talk explains why reactivity is a key requirement for openly authored, malleable substrates, and motivates fluid.cell, a reactive library designed to support the needs of these substrates. It preserves the good properties of widely available commodity reactive systems whilst expanding their domain. I explain the core reactive competencies of glitch freedom and early cutoff, and how fluid.cell delivers these competencies whilst supporting reactive graphs with bidirectional arcs, asynchronous propagation of reactive updates and allows the cause of these updates to be tracked to their source, and why these expanded capabilities are vital to support successful substrates. The talk finishes with a interactive visual demo of the reactive library on a simple test fixture.