The planarian flatworm is a simple animal with a mighty ability: it can regenerate itself from nearly every imaginable injury, including decapitation. Scientists have studied these worms for decades to better understand fundamental principles of natural regeneration and repair. One mechanism that is yet unknown is how organisms like these control the proportional scaling of tissue during regeneration. Now, molecular biologists have identified the beginnings of a genetic signaling pathway that puts the brakes on the animal’s tissue growth.