Jazz must swing — jazz musicians agree on that. However, even 100 years after the beginnings of jazz, it is still unclear what exactly constitutes the swing feel. With a sophisticated experiment and data analyses on more than 450 well-known jazz solos, physicists together with psychologists have unraveled a secret of swing. They were able to demonstrate that certain systematic deviations in timing are a key component of swing. These microtiming deviations are so small that they are not perceived as such even by professional jazz musicians, who nonetheless are using them unconsciously.