Rubber industries utilize many kinds of soft matter, polymer, colloids, surfactants etc. They are complex materials, but they all obey the law of physics, and their complex behaviors can be understood in terms of physics. In this series of lectures, I will try to explain the principles of physics which governs the complex behavior of these materials. In this first chapter, I will discuss the elasticity of rubbers: where does the unique elasticity of rubber come from, and how to understand the strange behaviors of rubber balloons.