I imagine that we all have those moments when the dividing line appears. Sometimes they’re major events, but often they’re just little ones. I remember the night in the shower when I discovered that the remaining sliver of an almost-used-up bar of soap could be “welded” to a new bar of soap, eliminating the awkwardness of having to use the flimsy remnant. I remember the day in the kitchen when I found that I could make a lasagna with noodles that hadn’t been precooked, and it would turn out just fine.

Although those two discoveries happened on my own, much of the time these life-changing moments involve other people. The one I remember best happened about 40 years ago, during a particularly frigid winter in New Jersey. I was telling someone — she could hardly be called a friend; she was more of a casual acquaintance — that I didn’t go outside unless I had to, because I found the intense cold so uncomfortable. “I used to feel that way too!” she said. “But then I got a down coat, and the cold didn’t bother me so much.”

I’d obviously heard of down coats, but it had never occurred to me to buy one. Nobody wore down In the time and place where I grew up, so its existence just wasn’t on my radar. But on this young woman’s advice, I bought a down coat — and sure enough, going out in the cold wasn’t torture anymore.

This may seem like a minor thing, but at the time, it brought about an unexpected epiphany: People affect each other’s lives.

Until then, I’d always imagined the world to resemble the illustration at the top of this post: People’s lives proceed in basically parallel lines. Sometimes the lines move closer together, sometimes they cross, and sometimes they uncross again. But I’d never really considered that when the lines cross, that crossing might alter the lines. As a result of my encountering another person, my life can change in some way. The line takes on a different hue.

For me, those encounters have most often happened at parties. There was the party in college where I casually mentioned that I had experience as a mime, and a couple of students who ran a campus theater offered me the chance to do a show. That led to the founding of the Princeton Mime Company, which lasted for another twenty years.

There was the party for faculty members of an electronic arts school, where a guy mentioned that a publisher was looking for someone who could write about Macromedia Director. That was the start of my lucrative stint as an author of computer books.

Then there was the wrap party after the taping of a public-access TV show, where I met a charming young woman who had just moved into town. I offered to show her around. Her name was Debra, and I ended up marrying her.

Not all of the line crossings are that momentous. During a visit to a friend’s house, Debra and I saw our friend pick up a banana and eat it upside-down, with the stem end in her hand. When we commented on it, she said, “That’s the way monkeys eat them. They’re easier to open and hold that way.” We tried it, and she was right. Since that day, we’ve both eaten our bananas upside-down.

Probably the most amazing of these life-altering line crossings are the ones where nothing really happens at all. You have a passing encounter with a stranger, and your eyes meet for some reason, and you remember that moment for the rest of your life.

Or perhaps your eyes don’t have to meet at all. In the movie Citizen Kane, the elderly ex-newspaperman Bernstein reminisces during an interview:

One day, back in 1896, I was crossing over to Jersey on the ferry, and as we pulled out, there was another ferry pulling in, and on it there was a girl waiting to get off. A white dress she had on. She was carrying a white parasol. I only saw her for one second. She didn’t see me at all, but I’ll bet a month hasn’t gone by since that I haven’t thought of that girl.

Even at the most depressed times in my life, the thing that always kept me going is the joy of unpredictability. You never know whose line is going to cross yours, and perhaps change it forever.