It's not every day you get to have an audience with the great Bill Gates. But on Monday, he Microsoft's famed co-founder took to Reddit for an Ask Me Anything session, in which he publicly answered Redditors' questions on everything from his guilty pleasures to choosing Microsoft's new CEO, Satya Nadella

Here are some highlights, edited for length:

If you were a current computer science student what area would you start studying heavily?

The ultimate is computers that learn. So called deep learning which started at Microsoft and is now being used by many researchers looks like a real advance[...]. It has already made a big difference in video and audio recognition--more progress in the last three years than ever before.

What is the worst case that you know of where your philanthropy backfired?

A lot of our failures have been backing science that didn't work out. One thing that is tough is when you think the government will take over something you start but they don't--we had that with a school lunch program. It might have been better if we hadn't done it.

You're working with startup TerraPower to bring low-carbon energy onto the commercial grid. What is the biggest impediment to bringing new nuclear technology onto the global market?

We need low-cost energy that is totally reliable. Most renewables will require storage which is expensive to do this. Nuclear will make a contribution if we can make it safer, cheaper, and better at dealing with waste. Terrapower has a design (on paper) that addresses all of these issues so now we are talking to countries about building it. It is a 4th generation reactor design that uses depleted uranium.

Any advice on how entrepreneurs should go about balancing business and philanthropy? Do they have to succeed first in order to give later?

Just creating an innovative company is a huge contribution to the world. During my 20s and 30s that was all I focused on. Ideally people can start to mix in some philanthropy like Mark Zuckerberg has early in his career. I have enjoyed talking to some of the Valley entrepreneurs about this, and I am impressed [at] how early they are thinking about giving back--much earlier than I did.

Can you describe your new role at Microsoft?

I am excited about how the cloud and new devices can help us communicate and collaborate in new ways. The OS won't just be on one device, and the information won't just be files--it will be your history including being able to review memories of things like kids growing up. I was thrilled Satya [Nadella] asked me to pitch in to make sure Microsoft is ambitious with its innovation. Even in Office there is a lot more than can be done.

How does Mr. Nadella's vision differ from yours and Mr. Ballmer's?

Satya is taking a fresh view of where Microsoft is--strengths and weaknesses. A new person gets to step back and change the focus in some ways. He is off to a great start.

What is your most expensive guilty pleasure purchase?

Owning a plane is a guilty pleasure. Warren Buffett called his the Indefensible. I do get to a lot of places for Foundation work I wouldn't be able to go to without it.

What do you believe is the next big issue that we need to address domestically?

Education would be the top issue since it is key to individual opportunity and to the country as a whole and we are not doing as well as other countries. After that I would say immigration since the injustice of the current system is incredible.

For more from Gates, check out the entire AMA here.