Can you tell apart identical twins easily? Identical twins are quite close to each other so it is hard to figure out who it is at a glimpse. Each twin, however, lives his/her own life, like classical particles having its own trajectory. We can consider the twin paradox in special relativity as an example, assuming that we cannot distinguish identical twins before sending one of them to space. After sending one of twins to space in a high-speed rocket, we can distinguish one twin on the earth from the other twin coming back to the earth, due to gravitational time dilation. In the same sense, we can also distinguish classical particles because they are also in macroscopic dimensions controlled by deterministic mechanics. Now the question is whether we can distinguish quantum particles too? The answer is in one of our papers, http://arxiv.org/abs/1212.5338v3 .