Online contests for open innovation – seekers posting innovation projects to which solvers submit solutions – have been developed into a new online commerce model. This study is one of the first to lift the veil of online contests. We identify that real world online contests are very different from what is assumed by previous studies. A real world online contest has uncertain number of solvers due to dynamic participation process. Feedback can encourage solvers to contribute more than the equilibrium effort. With a given award, if the seeker's feedback effort is high enough, the emerging number of solvers is a proxy measure of contest performance. By examining large-scale data from an online contest marketplace, we find that a contest with higher award, longer duration, shorter description, lower time cost, and higher popularity will attract more solvers. Specifically simple and ideation based projects are the most efficient in capturing solvers.