Selin gives blood for the first time. A nurse called Rosa puts a needle in her arm. A bag fills slowly. It has a letter on it: O. "Why a letter?" Selin asks.
"Because blood has names," says Rosa. "Yours is group O. Other people have A, B or AB. A doctor in Vienna found this in 1901. His name was Karl Landsteiner."
"He mixed drops of blood from his friends. Some blood mixed well. Some blood made small hard lumps. So he gave the blood names: A, B and O."
"Before 1901, giving blood was dangerous. Sometimes the person died. Nobody knew why. After 1901, doctors knew: blood must match. Landsteiner got the Nobel Prize in 1930."
The bag is full. Selin drinks a glass of juice. Her blood will help up to three people. "Now I know my name," she says. "O."