Menarche is the first occurrence of menstrual bleeding and it usually begins between the ages of 9–15. This makes menarche a crucial transition among other social, physiological and behavioural changes during puberty. In this soma-based research-through-design project we design an open-ended prototyping kit: Menarche Bits. The aim of Menarche Bits is to open a design space for young adolescents to create body-worn technologies that support them in making space for their experiences of menarche and trusting their menstruating bodies. Menarche Bits consists of heat elements and shape-changing actuators that can be worn directly on the body by adhering to the skin or being inserted into pockets in a stretchable fabric as part of a garment. We describe the soma design process behind Menarche Bits as an example of how body-worn technologies can intimately interact with the body and its movement, temporality and material changes.