It is a high bar to say that it's more fun than working on software because the work at Microsoft that both Melinda [Gates] and I did was thrilling. We were making breakthroughs and empowering people.

I believe innovation is the most powerful force for change in the world. People who are pessimistic about the future tend to extrapolate from the present in a straight line. But innovation fundamentally shifts the trajectory of development.

By helping us to be more productive, technology lets us to spend less time focusing on survival, and more on solving other challenges.

I was announcing to the public, in 2006, that I'd be leaving Microsoft in a couple of years and focusing full-time on the foundation. That was the time at which we went back to New York and Warren [Buffett] announced these gifts to a number of foundations, with a very high percentage of it going to us and basically doubling our capacity.

It's OK for China to invent cancer drugs that cure patients in the United States. We want them to catch up. But as the leader, we want to keep setting a very, very high standard. We don't want them to catch up because we're slowing down or, even worse, going into reverse.

There's only one trick in software, and that is using a piece of software that's already been written.

There are ecosystems like coral reefs [at risk] through ocean acidification. Those are valuable things that we should protect.