An amazing result in low dimensional topology is that only R^4 can have more than one smooth structure. Which means it is possible to find an ''exotic R^4'' which is homeomorphic but can not be diffeomorphic to standard R^4. Just like that the standard R^4 can be easily embedded in CP^2, we can also find an exotic R^4 just inside CP^2. In this talk, I will give an intuitive description of this ''monster'' with the help of some basic tools in 4-dimensional topology, like intersection form and Donaldson theorem.