President Obama sat down with three YouTube stars on Jan. 15, answering questions on topics like space exploration, race and women's health. (Peter Stevenson/The Washington Post)

President Obama has made a habit of answering questions on social media after his State of the Union address each year. In the past, he's answered top-voted YouTube question and hosted a Google Hangout. This year, like last year, he brought YouTube stars to the White House and gave them each a short interview. Here's what we learned.

1. The president is a huge space nerd

Obama lit up when he was asked a question about deep space exploration by Destin Sandlin, who has his own science-themed YouTube channel called "Smarter Every Day." In a 2010 speech at the Kennedy Space Center in Florida, the president promised to increase NASA's budget by $6 billion over five years and said he hoped the United States would embark on a manned mission to Mars in the 2030s. His enthusiasm for space travel was evident in his answer Friday as he described the possibility of a mission to land on an asteroid to research space travel and landing techniques ahead of a trip to the Red Planet. And Obama's favorite movie of 2015? "The Martian."

2. He had no idea that some states tax tampons as 'luxury items'

When Ingrid Nilsen, a YouTuber whose videos range from makeup advice to DIY projects, pushed Obama on the taxation of women's health products, the president mostly just plugged the Affordable Care Act and his efforts to get contraception covered by health insurers. But in perhaps the most surprising moment of the interview, Obama acknowledged he wasn't aware that tampons and sanitary napkins are taxed in the vast majority of states. Women who oppose the "tampon tax" say that they are being taxed for their periods. Obama did note, in fairness, that those taxes are levied at the state level, not the federal level.

3. He made a firm choice on which pants are appropriate for dogs

Responding to one of the most pressing questions of the past month, Obama weighed in on the now-famous "dog pants" controversy. (For the uninitiated, there are two choices. Only one is correct.) The president quickly chose the rear-legs-only pants, calling the four-legged pants "too conservative." Well, there you have it. The president has spoken.