I always like learning about how our bodies work. I am particularly surprised at how similar our genetic makeup is compared to other species. From February 2009's National Geographic (page. 70):
The notion of genetic switches explains the humiliating surprise that human beings appear to have no special human genes. Over the past decade, as scientists compared the human genome with that of other creatures, it has emerged that we inherit not just the same number of genes as a mouse--fewer than 21,000--but in most cases the very same genes. Just as you don't need different words to write different books, so you don't need different genes to make new species: You can just change the order and pattern of their use.
Perhaps more scientists should have realized this sooner than they did. After all, bodies are not assembled, like machines in factories. They grow and develop, so evolution was always going to be about changing the process of growth rather than specifying the end product of that growth. In other words, a giraffe doesn't have special genes for a long neck. Its neck-growing genes are the same as a mouse's; they may just be switched on for a longer time, so the giraffe ends up with a longer neck.
Isn't that incredible? The analogy comparing "words and books" to "genes and species" made the concept much easier to understand.