Sesamoid bones are small, flat bones that are embedded within tendons. To date, it has been thought that these bones develop within tendons in response to mechanical signals, but now (on p. 1831) Elazar Zelzer and colleagues challenge this assumption, focussing on the patella (the kneecap), which is the largest sesamoid bone in the human body. They show that that, in mice, the patella initially develops as a bony process that is part of the adjacent femur bone. Subsequently, they report, the patella is separated from the femur during the process of joint formation. This process is regulated by mechanical load; patella separation is perturbed in mutant embryos that are devoid of contracting muscle. The authors further demonstrate that, similar to bone eminences – superstructures that mediate bone-tendon attachment – the patella arises from progenitors that express the chondrocyte marker Sox9 and the tendon marker scleraxis (Scx) and that are regulated by TGFβ/BMP signalling. Together, these findings provide a new model for patella development and highlight that a high degree of plasticity exists during skeletal patterning and development.