Sticky ball object in box2d or chipmunk

How to create a sticky phisics object? In this flash game: http://bit.ly/1cZLVdu you drop something like a ball on other balls to make a tower from this object as high as possible. It looks like this balls have something like a “glue factor” and will stick together if placed quite in a stright line. How to create a similar game with box2d or chipmunk?