Left-handed pencils
It sounds like a joke. You can, of course, use a pencil with either hand.
The handedness of a pencil has nothing to do with its function; it’s about what you see when you hold the tool.
Nearly every pencil you find will have text that begins toward the tip and travels toward the eraser. For ninety percent of the population this means that the words are presented properly when they hold a pencil in their writing hand.
For the rest of us — the lefties — the words are upside down (except for when we erase).
It’s useful to consider: some things are so natural that they’re invisible. For most people, if the text on a pencil is noticed at all, it will be unremarkable. For others, it will be upside-down, as usual.
Of what other differences in experience are we unaware? What other design choices quietly favor one group over another? Surely, the list is extensive.