Want to stop spilling coffee on your walk? Oxford mathematician shares trick to avoid being a 'clumsy susan'

Spilling your coffee might seem like a clumsy accident, but science says it’s often about timing, not coordination. A mathematician from Oxford University has revealed a surprising way to keep your coffee from sloshing over the sides — the key, it turns out, is understanding how your hand and the coffee move together. The video was shared on YouTube by Oxford Sparks and features an explanation built around the work of Korean student Jiwon Han, who analyzed the physics behind why drinks spill while walking. To explain the problem, the researcher uses the analogy of a swing set in motion.

When a swing moves, it does so at its natural frequency, which is the rate at which it swings to and fro after being disturbed. Pushing it in sync causes its motion to grow — this is how the swing climbs higher and higher. This principle, called 'resonance,' applies to coffee as well! When you walk, your hand moves forward and backward at a speed of about 4 Hertz, thereby matching the natural frequency of coffee in a standard mug, which is also around 4 Hertz. That’s why the more you walk, the more your coffee sloshes about and eventually spills out onto your clothes and surroundings.

To stop that from happening, the researcher shows what happens when a swing is pushed out of sync. The back-and-forth motion gets interrupted, and the swing's length of movement rapidly decreases. Han applies this concept to coffee by suggesting people try walking backwards — or using what’s called the "claw grip" — to disrupt the pattern. Both methods reduce the motion of your hand to about 1.7 Hertz, which is no longer in sync with the coffee’s natural slosh frequency — this mismatch abates the waves in the mug and keeps more of your drink inside. Although the mathematician admits they’re sticking with a cup lid in real life, the demonstration can help those of us who religiously carry coffee on our person.

Viewers, especially beverage lovers, have questions and observations of their own. @danielhenderson7050 noted, "I discovered some years ago that simply raising and lowering the mug in time with your step will keep the coffee quite still. It's like the up motion compensates for the forward sloshing and vice versa." @CBMalloch remarked, "I learned by watching diner waitstaff that putting a spoon in the mug fixes it. The sloshing coffee swirls around the spoon, and the swirls damp its energy." @johnpenner5182 added, "May I suggest the Reverse Wag? ️You pitch back with the cup as you step forward, and vice versa, in sync with the gait of your walk. Like any music or rhythmic skill, it takes a little practice. I also tend to carry a cup with my hand over the whole top to bear the cup from above."

@Iacca said, "I walk slowly and more with my feet so my movement is smooth and creates very little resonance. I will try the claw grip." @JoeyRam commented, "I got here because over all drinks one can think, coffee is the one always spilled on tables and never cleaned up." @creepervideo1106 noted, "Always been using a claw grip (before seeing this video) since I started having coffee. Didn’t think it was weird, but other people told me it was kind of weird. I still use it, but only not when in front of guests."