Bill Gates took some time out today for his third Reddit Ask Me Anything(Opens in a new window) (AMA), where he fielded questions about Microsoft’s new HoloLens, what makes him feel dumb, and the world’s biggest challenges.

Redmond’s new hologram-like experience is “pretty amazing,” Gates wrote.

“Microsoft has put a lot into the chips and the software,” but it’s still just “the start of virtual reality,” he wrote. One of the main issues with VR is making sure users don’t get dizzy or nauseous, so “it will take a few years of software applications being built to realize the full promise of this.”

Gates, meanwhile, has something in common with Elon Musk: artificial intelligence (AI) kind of freaks him out.

“I am in the camp that is concerned about super intelligence,” Gates said. “First the machines will do a lot of jobs for us and not be super intelligent. That should be positive if we manage it well. A few decades after that, though, the intelligence is strong enough to be a concern. I agree with Elon Musk and some others on this and don’t understand why some people are not concerned.”

Still, he expressed an interest in working in AI. “When I started Microsoft I was worried I would miss the chance to do basic work in that field,” he wrote.

Right now, though, he is working with Microsoft on “Personal Agent, which will remember everything and help you go back and find things and help you pick what things to pay attention to.”

Another technology he’s approaching cautiously? Bitcoins. 

“Bitcoin is an exciting new technology,” he said, and digital currency is an area of focus at the Bill & Melinda Gates Foundation(Opens in a new window). But “we don’t use bitcoin specifically for two reasons. One is that the poor shouldn’t have a currency whose value goes up and down a lot compared to their local currency. Second is that if a mistake is made in who you pay then you need to be able to reverse it so anonymity wouldn’t work.”

The dark side of bitcoins is also a concern: “making sure that it doesn’t help terrorists is a challenge for all new technology,” he wrote.

Another personal challenge for Gates? Languages. “I feel pretty stupid that I don’t know any foreign languages,” he wrote. “I took Latin and Greek in High School and got A’s and I guess it helps my vocabulary, but I wish I knew French or Arabic or Chinese.”

Gates said he used the Duolingo, “but didn’t keep it up.” Perhaps a little healthy competition will inspire him to keep going: Gates said Mark Zuckerberg’s mastery of Mandarin is “incredible.”

Despite being able to rely heavily on our phones and Google for every answer, Gates does not believe that technology is making people less intelligent. “Technology is letting people get their questions answered better so they stay more curious,” he wrote. “It makes it easier to know a lot of topics, which turns out to be pretty important to contribute to solving complex problems.”

Among the complex problems Gates and his foundation are addressing are disease eradiction, a task that is made much easier when people have a sanitary way to dispose of waste. Gates recently sang the praises(Opens in a new window) of a machine that turns feces into drinking water. It might sound gross, but Gates said it tastes “just like drinking any other kind of water, except that people get a little freaked out by the whole idea.”

In third-world countries, “sewage is a problem,” Gates said. “Since it costs money to process, it just gets dumped in slums in poor countries. The system the rich world uses of pumping in clean water and pumping it to a processing plant is too expensive. I challenged engineers to create a processor of sewage where the costs could be covered by the energy and water (clean water) that it outputs. We have made progress on that. One team, Janicki, which was written up in Wired, is send[ing] a prototype machine to Senegal later this year. Getting rid of sewage helps a lot to reduce disease and improve living conditions.”

For now, Gates pointed to vaccines as the innovation that has most improved life in poor countries over the last five years. “Being able to grow up healthy is the most basic thing,” he wrote. “So many kids get infectious diseases and don’t develop mentally and physically. I was in Berlin yesterday helping raise $7.5B for vaccines for kids in poor countries. We barely made it but we did, which is so exciting to me!”

His focus now is on Polio eradication. “Our last case in Africa was 6 months ago and we are hoping no more show up. It takes over a year to be sure,” he wrote. “We still have cases in Afghanistan and Pakistan. Until we get rid of it there it can spread back to other countries. Pakistan is starting to take this seriously including the army and the government. They need to do the same things that were done in Nigeria. The Taliban makes it very difficult. They have killed women going to vaccinate kids many times.”

For more, check out Gates’s AMA from February 2014 and 2013.

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