Toward the end of seventh grade, my middle-school band took a trip to Cedar Point, which was pretty much the theme park to which midwestern middle-school bands traveled. (I imagine it still is.) They had this indoor roller coaster there, called the Disaster Transport. My friends and I were standing in line for this roller coaster, winding up the dimly lit cement steps, when we turned a corner and came across a huge pile of money.

We picked it up and counted it; it was a very specific amount of money. I don’t remember now exactly how much, but for the purposes of this retelling, let’s say it was $134. That sounds close.

We had barely had time to whiplash from marveling at our good fortune to guiltily suggesting we should find somewhere to turn it in before a group of older kids ahead of us snatched the cash wad out of our hands. They claimed it was theirs; it was not theirs—they counted it in front of us and exchanged “Whoa”s and high fives. We were hapless, gangly middle schoolers (I was growing out my bangs; it was a rough year). They were confident we would do nothing to stop them, and they were right. So that was the end of that.

Until, Part Two:

A little more than a year later, I went to a summer program at Michigan State University, a nerd camp where you take classes like genetics for fun. One evening, as we were sitting around in the common area, chatting and doing homework, I overheard a kid telling his friends how he’d lost a bunch of money last year at Cedar Point.

With very little attempt at chill I interrupted their conversation and grilled him on the particulars.

Was he there on May whatever date I was also there? He was.

Did he lose the money in line for the Disaster Transport? In fact, he did.

How much money did he lose? $134, exactly.

* * *

Though “What are the odds?” is pretty much the catchphrase of coincidences, a coincidence is not just something that was unlikely to happen. The overstuffed crate labeled “coincidences” is packed with an amazing variety of experiences, and yet something more than rarity compels us to group them together. They have a similar texture, a feeling that the fabric of life has rippled. The question is where this feeling comes from, why we notice certain ways the threads of our lives collide, and ignore others.

Some might say it’s just because people don’t understand probability. In their 1989 paper “Methods for Studying Coincidences,” the mathematicians Persi Diaconis and Frederick Mosteller considered defining a coincidence as “a rare event,” but decided “this includes too much to permit careful study.” Instead, they settled on, “A coincidence is a surprising concurrence of events, perceived as meaningfully related, with no apparent causal connection.”

From a purely statistical point of view, these events are random, not meaningfully related, and they shouldn’t be that surprising because they happen all the time. “Extremely improbable events are commonplace,” as the statistician David Hand says in his book The Improbability Principle. But humans generally aren’t great at reasoning objectively about probability as they go about their everyday lives.

For one thing, people can be pretty liberal with what they consider coincidences. If you meet someone who shares your birthday, that seems like a fun coincidence, but you might feel the same way if you met someone who shared your mother’s birthday, or your best friend’s.