A fixed mindset is debilitating because it prevents teachers from removing the low-level goals that take time and energy away from the things that we really need to be focusing on. Perhaps doing a life science unit in the spring can be challenging, but the challenge is the goal we set, not the life science unit itself. The life science unit is just a means to an end, and it may or may not be appropriate.
If the task is appropriate, we can keep it as a thread of achieving that goal. If it's not, let's abandon it. That's part of the growth mindset process that brings a level of flexibility to designing science curriculum. Abandoning higher-level goals, on the other hand, is something that we should be really stubborn about.
This brings us to challenge, and challenge is where rigor comes in. Angela Duckworth defines rigor as challenge that exceeds skill, which is an opportunity for growth. Flow, on the other hand, is where challenge meets skill, and is an opportunity for practice. These ideas are directly applicable to the Next Generation Science Standards and the kind of environment we need to create in the classroom.
It's important to be careful when it comes to rigor, though. Many teachers will define rigor as assigning a lot of reading or performing flashcard sets dozens or hundreds of times. This is not rigor; this is learning by rote, and its results are dubious at best. Rigor is challenge that exceeds skill. Flow, the place to practice skills already learned, might seem beneficial, but often times it's really just a place where students can lose track of time, retreading skills they've already learned, and that becomes old hat. While flow may have some place in science teaching, it should not comprise the majority of science time. Most of the practice can come within the opportunity for growth.