Take a glass that is partially filled with water and drop a cork into it. The cork won't float in the center, but keeps drifting to one side and attaching itself to the side of the glass.

\-------------------/
 \                 /
  \##-------------/
   \             /
    \           /
     \         /
      \_______/

Without swirling the water, how can you make the cork float in the center?