This week my 15-year-old son has been doing "work experience". A lot of new experiences for him: travelling into Central London on his own; spending the day with adults he hardly knew; a much longer day than a normal day at school, with an entirely different rhythm of work.
One lunchtime he met a schoolmate along the street, who was doing his work experience at a hospital in another part of London. (His father is a doctor.)
My first thought was: how unlikely is that? I have been to Central London many thousands of times, and I have hardly ever met people I knew in the street. (Not counting people working in the same building or heading for the same event.)
But not so fast. The streets are full of people, but most of them are adults. Although there are lots of kids on work experience this week, these only amount to a tiny fraction of the total street population. Some more perceptive adults may notice that there are more kids around than usual; meanwhile the kids possibly pay more attention to each other than to the adults. So if there are two 15-year-old boys in a crowded street, they are perhaps more likely to see each other than two 45-year old men would. Just a theory, but it would help explain the improbable event of my son's meeting his schoolmate.
Of course, we don't always have to find an explanation for something just because it seems unlikely. But an unlikely event often stimulates this kind of enquiry. Note how my hypothesis required me to creatively imagine how a crowded street in Central London might look to my son.
Perhaps more generally, people who have something in common (born in the same village, similar vices, or whatever) have a higher than random chance of meeting in a crowd. Or it might be that we only remember those meetings that have a greater significance. Fiction often picks out such meetings, and some authors build elaborate plots around unlikely coincidences; but we may not wish to take fiction (or human memory, which is sometimes barely distinguishable from fiction) as a reliable guide to the significance of everyday life.