An 70-year-old married a 20-year-old. A year after the wedding the couple arrives at the delivery room.

A male boy weighing 3.5 kg was born, healthy and beautiful. The nurse asks the old man "Yours?" "Yes," the old man replies proudly.

"Congratulations," the nurse replies. "Well…" says the old man, "the old engine still runs!"

Two years later the duo arrives at the delivery room again. A 3.5 kg girl was born, healthy and beautiful. The nurse asks the old man: "Yours?". "Yes" the old man answers. "Well done" the nurse answers. "The engine still runs!" said the old man.

After two more years, they come to the delivery room again. A 3.5-year-old male son is born, healthy and beautiful. The nurse asks the old man: "Yours?". "Yes," the old man replies. "Congratulations," the nurse says, "that’s really impressive." "Well…" says the old man, "the engine is STILL running!"

"Well… says the nurse, "you may want to change the oil, the last one came out black."