Imagine if it were no easier to recognize a face than to identify a rock. Face-Blindness (Prosopagnosia) affects 2 of 100 people, making it hard for them to recognize faces, even of people they know very well.
Cecilia Burman makes this unusual condition easy to understand in a funny and enlightening essay explaining some of the challenges we might have learning to recognize stones and how we might overcome those challenges.
Easy to recognize in his usual spot on the steps:
Harder to spot if you unexpectedly see him in the garden: