Newbie question about attaching dynamic objects to moving objects

I know this is an insanely stupid newbie question but I can't find enough documentation to help me figure this out. My question is: how do I attach objects to one another dynamically? For example, if I have balls falling out of the sky and I catch them in a bowl (via collision detection), I would like them to be stacked one on top of another. The only movement is horizontal and I would like the balls to stack and move horizontally with the bowl.

Currently, I can only get the balls to fall out of the sky but they don't 'stick' to my bowl when i move it sideways (on the x-axis).

Please help as I'm not sure how to approach this problem.

Thanks,
Kevin