Why does coffee spill when you walk?
Walking with coffee: Why does it spill?
Your walking rhythm happens to match the natural sloshing frequency of coffee in a mug.
Ig Nobel 2012
Researchers filmed people walking with mugs of coffee. A typical mug’s size means the liquid naturally sloshes at almost exactly the frequency of our steps, so each step amplifies the wave.
Tips from the paper: walk slowly, watch your mug (it helps you smooth out), and don’t fill it to the brim.
Your coffee is resonating with you. Romantic, and messy.