What challenge facing the world is so difficult it could make a billionaire feel powerless?

One Reddit user posed this question to Microsoft co-founder Bill Gates during his fourth AMA (Ask Me Anything).

Last month, Bill and Melinda Gates published their two-part 2016 annual address—titled “If You Could Have One Superpower, What Would It Be?”—which outlines the need for an “energy miracle” and a reform to current norms of women spending a disproportionate amount of time doing unpaid work.

Early Tuesday morning, Bill Gates stopped by Reddit to answer questions about the global challenges described in his letter—though the affair wasn’t without levity. To mark the occasion, Gates released an animated teaser, in which he shares a boat with Reddit’s alien mascot Snoo:

He also recreated his high school yearbook photo, as a cheeky photo to prove that it was, in fact, Bill Gates answering users’ questions (note the Reddit username subtly placed in the background of the “after” photo):

Over the course of the AMA, Gates answered over 30 questions from users. Here are a few of the highlights.

What Makes a Billionaire Feel Powerless?

What Will Humans Accomplish by 2036?

Why Isn’t He Running for President?

On Climate Change Politics

Should Artificial Intelligence Be Regulated?

Thoughts on the Apple vs. FBI Debate?

Best College Memory?

On Dancing

Worst Recent Invention?

Best (Future) Invention?

Plans for Retirement?

Before signing off, Gates shared a video with redditors to express his gratitude and to spread a simple message: “Please use your superpowers for good.”

To read all of Gates’ answers to redditors’ questions, check out the original AMA discussion. The full text of Bill and Melinda Gates’ annual letter—”If You Could Have One Superpower, What Would It Be?”—is published on Gates Notes.