Today Is 101010: The Ultimate Answer to the Ultimate Question

Today is October 10, 2010. 10/10/10. In binary, that's 42. And 42 is The Answer to the Ultimate Question of Life, the Universe and Everything. Or at least, that's what Douglas Adams says.

Many people wonder what Adams exactly meant by 42, the answer given by the supercomputer Deep Thought in The Hitchhiker's Guide to the Galaxy. Why did Adams pick that number?