> 
> **Possible Duplicate:**  
> 
> [Vortex in liquid collects particles in center](https://physics.stackexchange.com/questions/3244/vortex-in-liquid-collects-particles-in-center) 
> 
> 
> 

Why when you mix a cup of tea with a teaspoon, tea-craps collect in the center of cup, instead of goes near the walls?