Teaching concepts through analogies

I taught convection the other day in a 6th grade (urban classroom) . The inquiry activity was to design a house with passive ventilation. If the windows and doors were placed correctly, the hot air (from incense sticks) would gush out, allowing cool, fresh air to enter the house and generate a steady convection current.
Surprisingly it was extremely hard for the students to understand why hot air rose and cold air sank. They could parrot the sentences out, but couldn’t explain why.
I was slightly taken aback and at the spur of the moment came up with an analogy that didn’t really work too well.
Later I realized that I really needed a resource I could quickly go to and grab an analogy for the concept I was going to be teaching. Sometimes, in the thick of things, its hard to keep a clear mind and unless you have these explanations completely internalized, its not always possible to draw on them.
If we had a wiki of simple, real-world, analogies, then on the day before the class, I could just go to it, search for the topic, print out the relevant analogy and teach it correctly.

So, I started one! If you are interested in contributing, improving or using it, please go to this Wiki (Analogies for teaching Science).