The plasma membrane serves as a major hub for signal cascades to control crucial cellular processes. But it is a fluid medium, which makes the signaling processes difficult to monitor. Now, German scientists have designed a molecular “paintbrush” technique to trigger, control, and also monitor signaling processes. As they write in the journal Angewandte Chemie, their modular system made of light-activatable molecular building blocks can, for example, induce patterned contraction inside living cells.