build solar eclipse viewing boxes. But before doing that, you may want to have your students review how eclipses happen. The following three videos are good for that purpose.
Of the three videos in this post, this one from Vox is the most detailed and is well suited to use in a flipped lesson tool like EDpuzzle.
If you need a short refresher to post in a classroom blog, the following video from the Washington Post is a good choice.
Finally, for elementary school classrooms, SciShow Kids offers the following lesson about solar eclipses. The shortcoming of this video is that it is based on the solar eclipse that covered parts of Europe in 2015. That might be a little confusing for some students.