One of my favorite ways to teach is through error and frustration. Maybe that sounds mean, but one of the most "a-ha" activities that we do every year results in just that - mistakes, failure and frustration.
Before teaching our unit on Classification of Living Things, I decided a few years ago not to tell the kids what we would be learning next, but to just give an activity with easy-to-follow directions, but no guidance about why we were doing it.
I created a few identical decks of cards with pictures of forty-some animals on them. Each group gets a deck of cards and is given the task to dividing the animals into two groups based on their characteristics. Then, each of the two groups must be divided again into two groups. They record their results on a sheet like this:
What the students do not realize, however, is that the groupings they create and the records that they keep are not really the goal of the assignment - it's the discussions that arise during the activity. Inevitably, every system that the students devise during this process becomes a horrible failure. Some students opt for the first two categories to be land and sea, but cannot decide about the frogs. Some opt for big and small, but then struggle with the medium sized animals. Some opt for furry and not furry, but struggle with classifying the birds and whales.
In the end, we have a whole-class discussion about some of the problems involved in their classification schemes. Every year that we have done this activity, the students have drawn the same conclusions about the difficulty of classifying organisms - which characteristics are important and really show evolutionary relationships, and which ones are not.
Even though our textbook surprisingly does not describe the relationship between evolutionary relationships and common ancestors to classification, the students begin to make these connections during our conversation. We just finished learning about evolution, natural selection and common ancestors, so the tie-in fits perfectly.