Ever since the existence of molecules was proven and molecular reactions were predicted, humans have wanted to visually observe how such events proceed. Such observations of single-molecule reactions are highly important for the fundamental understanding of chemical sciences, which would aid in the development of novel catalysts, materials, or drugs, and help us decipher the complex biochemical processes. However, this was not possible for the longest time in modern chemistry, and so far the information of dynamical processes on the nanometer scale was obtained only from indirect methods because molecules were too small to be visualized.