How do whirlpools form?
Any time water flows through a narrow path, it forms at least a partial whirlpool. As the water passes through the narrower opening, it accelerates and forms a more powerful force. If the downstream area then enlarges, it can mature into a complete whirlpool.
As water is pulled into an opening by gravity, it begins to spin. (See below for more information about the direction it spins.) Once this begins, it intensifies and forms a cavity in the center of the drain. The cavity creates a vacuum into which objects such as bubbles, water molecules, and other floating objects are pulled. As these objects are “sucked” into the vortex, the centrifugal (outward) force maintains the hole in the middle through which air passes.
What makes whirlpools spin?
As the water is pulled down into the opening, the water particles fight for the smaller space and push each other to the side. This pushing and nudging by itself would not necessarily cause the water to spin, especially in a perfectly-shaped funnel and no other directional influence on the water. But a perfectly-shaped funnel never exists in nature. There are inconsistently-shaped rocks or other obstructions that force the water away from them. This initiates a spinning motion that accelerates as the water is pulled by gravity.