Don Delprete, age 10, of Staten Island, N.Y., for his question:

WHEN WAS THE FIRST BLOOD TRANSFUSION?

Back in 1667 a French doctor successfully transferred lamb's blood into the arm of a feverishly ill boy. Later attempts at blood transfusions failed, leading to laws preventing further experimentation. Although a few blood transfusions were successful before the 1900s, widespread use of this lifesaving technique didn't begin until the early 1900s when Karl Landsteiner, an Austrian American physician, discovered blood types.

Landsteiner's discovery made it possible to match the blood type of the person whose blood is used (the donor) to the person who receives the blood (the recipient). In the early years of blood transfusions the donor came to the recipient's bedside, where the blood was transferred directly. Today, various preserving solutions permit blood to be stored in facilities called blood banks.