When my Microsoft co-founder, Paul Allen, and I were kids, we fell in love with computing. But software wasn't the first thing we bonded over. It was Robert Heinlein.

I met Paul around the time I had finished reading all of the science fiction writer’s early books. Those novels were adventure stories with titles like Rocket Ship Galileo and Space Cadet. They weren’t labeled children’s books, but they appealed to kids. The plots were very straightforward. They always had a simple moral and involved a little bit of cool technology and a little bit of romance. I loved them.

Then, when I was in seventh or eighth grade, I got into the Heinlein novels that were meant for adults—but I didn’t know it. Starship Troopers was set in the future but drew parallels with the Cold War. Stranger in a Strange Land and The Moon Is a Harsh Mistress went even deeper into Heinlein’s philosophy of life and his concerns about the future. They were dark and ambiguous. You didn’t always know who the hero was. "This is not the Heinlein I’ve been reading,” I thought. “What happened to the guy?”

I met Paul around the same time, and we got to know each other by talking about sci-fi. I thought I had read a lot of it, but Paul way outdid me. (To be fair, he did have the advantage of being two grades ahead of me in school.) I had one bookshelf filled with science fiction. He probably had eight. Paul explained that the Heinlein books I had stumbled on weren’t children’s stories—they had messages and were supposed to help you think about the real world. That was news to me as a young teenager.

Eventually we started looking for other shared interests. Our school acquired a computer, and we said to each other, “How does this thing work? Let’s try and make it do something.”

Of all the sci-fi I read as a teenager, Stranger in a Strange Land is my favorite. It was published in 1961 and is Heinlein’s most popular book. It’s about a human named Michael Valentine Smith, who’s raised on Mars by Martians and then returns to Earth as a young adult. Because he grew up on Mars, he has psychic abilities and is super-intelligent. After some early adventures, including an escape from the facility where he’s being studied by scientists, he becomes fascinated by the world’s religions. In the novel’s futuristic setting, religions are more politically powerful than they are today, and Smith decides to start his own.

He calls it the Church of All Worlds, and through it, Heinlein predicted a lot of the hippie culture that was to come later in the 1960s. Smith’s adherents learn to “grok” things, a Martian term meaning to understand something by becoming one with it. (The idea of grokking got picked up in popular culture and became, at least for a while, a term you heard a lot even outside the context of the book.) Smith’s followers live in communes, which struck me as pretty out-there when I read it as a teenager.

I love sci-fi that pushes your thinking about what’s possible in the future. In Heinlein’s case, hippie culture isn’t the only thing he predicted. Among other things, Stranger in a Strange Land and other works of his mention what he called a “hydraulic bed”—what we now know as a waterbed. He also does the classic sci-fi thing of using an obviously fictional setting to ask profound questions about human nature.

Heinlein isn’t known as a particularly humorous writer, but Stranger in a Strange Land definitely has some funny parts. Early in the novel, for example, a nurse offers Michael a glass of water. To her, it’s a simple gesture, but it has a lot of meaning for him because water is so scarce on Mars. He thanks her: “May you always drink deep.” After they both take a sip, she can’t figure out why he “seemed content to sink back, as if he had accomplished something important.”

I’m glad I stumbled on Stranger and Heinlein’s other grown-up novels when I did. Everything I had read before them had a tidy ending. Here though the ending is unclear. It’s up to us to decide what happens next, just like in real life.