Lillian was getting undressed for a bath.
“Lillian, look you silly – you’ve got your shoes on the wrong feet!”
“No I haven’t…”
“Yes you have,” I reassert dogmatically, removing the shoe.
She takes the shoe and holds it up to her foot – sole of shoe to sole of foot.
“See Daddy! Match! I haven’t did it on the wrong feet!”
I had to admit, she had found a match – and a reasonable basis for her indignation.
Lillian doesn’t quite discover, but trips up on, mirror-image asymmetries.