译文：过去的时间逝去了不可追回，今天的日子也会在转瞬之间过去。赏析：已经过去的就让它过去，不必再追悔。我们要活在当下，珍惜每一天。与其用泪水悔恨昨天，不如用汗水拼搏今天。

Natural experiments are a powerful tool for understanding causal relationships in the real world.

Sometimes, the hardest battles are fought within the confines of our own minds.

The thing about happiness is that you only know you had it when it's gone.

我不是一个放弃自己声音的人。