Look for the taurus in the picture, see how it is 'pinched' in the middle?  can you imagine a pshere with teh same thing happening (like a balloon with to two small sticks pushing in from eithe side')

See picture here:



kind of like a particle vacuum, where the center of it acts as if it is sucking up gas molecules, or water molecules being drawn towards the center.

There are other pictures there as well.