The Internet of Things (IoT) makes it possible for tiny devices with sensing and communication capabilities to be interconnected and interact with the cyber physical world. However, these tiny devices have limited computing power and memory, so they often cannot run commodity operating systems, such as Windows. and Linux. IoT devices are deployed everywhere, from smart home appliances to self-driving vehicles, and their applications impose ever-increasing and more heterogeneous demands on software architecture. There are many special-purpose and embedded operating systems built to satisfy these wildly different requirements, from early sensor network operating systems, such as TinyOS and Contiki, to more modern robot and real-time control systems, such as FreeRTOS and Zephyr. However, the rapid evolution and heterogeneity of IoT applications calls for a different solution. Specifically, this work introduces Tinkertoy, a collection of standard operating system modules from which developers can easily assemble customized operating systems. A customized operating system provides precisely the functionality needed by an application and consumes up to four times less memory than other IoT operating systems without sacrificing performance.