One day, a young mother brought her two sons to visit our garden. The boys, who looked about four or five years old, sat in a large trolley by the marine fish tanks. They were identical.
"Are they your sons?" I asked the lady.
"Yes!" she replied happily and with pride.
"Are they twins?"
"Yes!" she said, her good mood unchanged.
After a moment of silence, I decided to make a joke. "If they're twins, why are their eyes different? One has black eyes like mine, and the other has blue eyes like yours."
"I know! But I don't know why!" she answered with a smile.
The two boys looked at me strangely.