Sphere cells suspension culture - (Sep/13/2005 )

I have some sphere cells in neurobasal medium.After 4 days the cells starts clumps together and form bigger spheres.
But i dont know why they are forming sphere cells flotting on the medium.
small spheres again attaches and form bigger spheres.
why it is so?

Neurospheres will form and floating around in neurobasal medium (+N2, B27) because it's serum-free. If you add some serum they will attach.

I would like to know why theses are floatting?
what couse it to form spheres?