"Playing with Puffer fish" is an iPhone app designed by tomoko shigetomi, offering a delightful experience of interacting with cute Puffer fish. The app allows users to raise their own Puffer fish and engage in activities to improve its mood. By sliding the screen, users can simulate playing with the Puffer fish, with its mood enhancing as interaction progresses. Feeding the Puffer fish is crucial to maintaining its mood, requiring users to use tweezers to feed it in a realistic manner.

As users continue to engage with the Puffer fish, they can earn points that unlock background images. Even when the app is not in use, the Puffer fish continues to grow, providing an enjoyable experience for users to return to.