How often it happens — you can walk past something for years and never think twice about it or even notice it at all. Then, once it is called to your attention, you begin to see it rather often.
That happened to me (again) quite recently. I read an article about the teeth found around the edges of certain kinds of leaves. For instance, alder leaves always have noticeable teeth, and even cottonwoods have small teeth evident upon close inspection.
For years, I had known about teeth on the margins of alder leaves, but had I thought about them?
Hmmmmm ... no.