Unit studies are popular with many homeschoolers. When I first began looking into using a unit study approach to teaching my children, I became overwhelmed with all the choices that were out there. If you want to study airplanes, you could find books and books on the subject - broken down into all the different subject areas - history of airplanes, the science of airplanes, stories of pilots and airplane lore, you could even figure out how to make your own airplanes - wooden or paper- and use math to calculate how far they could fly. If you really are good at researching you could probably make a unit study on airplanes last a whole year.
That's where the overwhelming part came in for me. Not only was it overwhelming trying to decide what to use and what not to use, there was the children's interest level to consider. Some children could be fascinated by such a subject for many weeks. Whereas others could be bored within a day.
Over the years I've come to realize that my children learn best when they instigate the learning. The penguin phase is a good example. One day while driving to work, my 4yo son started asking me questions about penguins. Where do they live? What do they eat? Can they fly? I answered the questions as best as I could and made a mental note to see what books we had on penguins at our office (one of the advantages of being self-employed, I can take my children to work with me).
We have a well stocked library and I was able to find several Usborne books that gave information on penguins. So we sat and read through these books. Next he wanted to colour some pictures of penguins, so I went on line and downloaded several penguin pictures from http://www.enchantedlearning.com . As the days went by and his interest in penguins didn't wane, I began looking for other penguin sources. I found the cartoon video, The Pebble and the Penguin which quickly became my children's favorite. We bought the book Mr. Popper's Penguin and read that to him. We provided lots of paper and markers for him to draw his own penguins with. Soon he had his 2yr old sister drawing very close likenesses of penguins too. In their play they were penguins and they often built nests out of the couch cushions so that they could sit on their eggs. I became the zoo keeper and had to feed them tuna fish everyday for lunch.
Lucky for us, the March of the Penguins movie came out about this time so even though this was a documentary and not my children's favorite type of movie, we went to the theater and my then 5yr old and 3yr old sat enraptured throughout the entire movie.
Over the course of weeks, my children learned what penguins ate, how they cared for their young, where they lived and where that was in relation so where we lived, the anatomy of penguins and the different types of penguins.
From penguins their interest turned to foxes and we duplicated what we did with the penguin craze. Foxes were soon overturned for egrets, then birds in general, followed by tigers. I have tried to document their learning by keeping samples of their art work, writing down the titles of books read and movies watched, and by journaling some of their more interesting projects. All of this information can then be incorporated into a notebook or portfolio.
What I have learned while doing this method of unit studies is that by following the child's interest, and not making it into a big "we have to learn something" project, my children have retained the information learned and it hasn't been a big stressful endeavor. Having lots and lots of books on hand makes it easy to follow this kind of approach. When my child becomes interested in something, we need only turn to our library shelf to find a book that covers that subject. And, of course, a good high speed internet connection and heavy duty printer is helpful as well!