This work illustrates and reflects on the design process of MLTK01, an open-source toolkit for fast prototyping tangible learning things, built on top of Arduino and ml5js. The toolkit was developed as a response to the current lack of fast and easy to use tools for tangible experiments with machine learning. Learning from insights gained through previous projects, we defined a set of basic building blocks necessary to enable such experiments and engaged in an iterative process of sketching, prototyping and preliminary testing of the toolkit. MLTK01 includes a custom PCB, a software library and accessories. Together with a descriptive account of the design process we also discuss possible applications of the toolkit and its implications for a design process of tangible learning things@en