Sil-X stage balls are partially filled with liquid silicon. The liquid center gives the ball a nice weight and a low center of gravity making arm/body rolls and stalls easier than standard stage balls.
Foot and neck stalls and head balances are also facilitated by the stability of these balls.
The 78mm Sil X's are a great middle ground; they are big enough to be good for body rolling and small enough to be used as stable juggling balls.
The SilX ball still has a little bit of squish to it, making catches nice and comfortable.
Warning: If stored incorrectly these balls can become misshapen.
Warning: The white balls should be stored separately as they can take on the color of other balls/objects they comes in to contact with.