This mobile is a beautiful example of the grasping and batting practice that infants need. If you take the Montessori course for this age group, it is required homework. Not too hard to make, but this one came out really well!
The idea behind the mobile is that you hang it within reach of your child, so that he or she can watch it, track it visually, and gain hand-eye coordination by batting at it in an attempt to grasp the tantalizing balls.
This video is from Goosedesigns on Etsy. I just went to her store to try to order a mobile at the last minute, and she's busy, so I will have to shop in advance next time. Got the ring and strap from her for a gift and it was a lovely set.