"Test your knowledge" exercises between days

A student suggested that we have some "challenge" slides that they could complete on their own to solidify their understanding. This would be nice to put between the days. For example, some questions for day 1 could be "what's wrong with this if expression", or "what's the type of `x`".

I think we should make these clearly optional (and if instructors have additional time on any day, they can go through them during class even). They should be safely skippable.