You're at a party. Somebody hands you a glass. Hey, come on over here! Somebody else shouts. You take a step and the next thing you know you are wearing that tasty beverage.
But why? When we walk, we keep our torsos straight, don't we? Walking feels entirely steady and constant. So why is it so hard to carry a liquid?
If you doubt the difficulty of this exercise, we suggest you try filling up a bucket with water and carrying it at chest level without spilling any. But try it outside first!
The answer is that although we feel ourselves to be quite steady when we walk, in truth that is anything but the case. When you walk your torso bobs in all directions.
Walking is a complex muscular phenomenon that amounts to allowing yourself to fall slightly forward and then preventing yourself from falling completely by extending your foot. The extended foot halts the motion which was begun partly by gravity and partly by the pushing of the other foot. Now the torso moves over the base of the resting foot which, once the torso has passed it, begins pushing it into a forward fall again until the other foot arrests the motion.
On top of this, we must also constantly prevent ourselves from falling sideways by flexing and releasing the gluteus muscles around our buttocks and hips. That's why people sway as well when they walk.
This is what walking is: not the smooth, undifferentiated motion that it seems to be, but an ongoing series of up-down, start-stop, and side-to-side maneuvers. No wonder drinks fly!