There's a strange habit that develops in scientific life. Well, to be fair, there are a lot of strange habits that develop, but I'll focus on just one today. It's a habit of questioning everything, and I mean everything.
It's a part of the training to become a scientist, but it's not one you learn through any class. Instead, over the course of years you begin to recognize that what at first feels like harsh, personal critique is actually a vital part of the scientific process itself.
See an obvious mistake? Call it out. Don't understand something? Ask the question. Spot a hidden assumption? Bring it to the surface. Hear a conclusion asserted without evidence? Challenge it. Find a sacred cow? Turn it into hamburgers.
The practice of questioning everything is necessary, because the game of making science actually work isn't something done personally - you need a community. Sure, you might try your best to limit bias or carefully control your experiment or make the best assumptions in the math or apply the proper statistics, but you absolutely rely on your colleagues to check your work. Science is a process that thrives on that constant ever-vigilant critical examination from your peers, from your advisor to your research group to your journal referee to your presentation audience.
The rules of this game are simple: challenge everything.