Simple but lovely little dynamics app; Turn Around, created by coolap. There isn't much word on the dev or previous work and it appears that the blog has been set-up just for this app. In the app, you tap to add point sprites and they will start moving around and colliding with one another. At an appropriate distance a small sprite may be captured and will keep turning around a bigger one depending on the hierarchy of small vs larger sprites. Demo (lite) version is also available.