One sunny afternoon

In a bus shelter, one sunny afternoon, I was tying my shoelaces, which had come loose several times that day. As I tied the second pair with a single knot, I looked up to see an old lady, who evidently had been watching me.
Why don't you double-knot your laces? Don't you know how? My mother taught me to do that when I was a little girl!
I smiled. Now I am fifty, I thought. No one had taught me to double-knot my laces when I was a child. Why not? I wondered. And why had I not noticed others doubling their knots? When I was in my twenties, someone remarked that he thought I was an orphan.
But the old lady, who was at least my mother's age, had taught me after all. Now, I double-knot my shoelaces. They haven't come loose for many years.