I was watching a Richard Dawkins video (The Genius of Charles Darwin) with both my daughters. Dawkins was discussing evidence for evolution and was talking with an eye expert. Thinking I knew what was coming, I stopped the DVD and had my daughters discover their own blind spots by closing one eye, then extending their fingers and staring at the finger opposite the open eye while moving the other finger closer. Eventually the moving finger passes into the blind spot and seems to disappear. I explained that this happens because our retinas are actually backward, yet our brains are able to fill in the missing piece of our visual field.
Then I turned on the DVD again and Dawkins went through the same experiment, explaining that this was very good evidence that the eye evolved.
More than an argument for evolution, what I hope my daughters get from that is that the simplest experiments you can do yourself really can open you up to the deepest and most amazing scientific ideas.
It was a good day.