SignificanceMicroscopic gas–vapor bubbles play an important role in various processes in nature, industry, and medicine. Sometimes they are desired: to lower the temperature at which a liquid starts to boil or to enhance the contrast in medical ultrasound applications. In other cases, bubbles are harmful: they cause damage when expanding and collapsing near the propeller blades of ships, and sickness upon expanding in the veins of plants and animals. In spite of their ubiquity, the “nucleation” of bubbles, i.e., the mechanism responsible for their initial formation, has remained mysterious for a long time and, even today, it is not fully understood. Here, we show that bubbles can nucleate when two solids are gently rubbed together in a liquid: “tribonucleation.”