"It was so surprising," University of Pennsylvania immunologist Steven Reiner says, "because it was the simplest solution."

The puzzle that Reiner's team set about solving: How do we retain and even strengthen immunity to any given disease when our infection-fighting lymphocite cells - only one is programmed for each pathogen - are known to die in the first bout?

The answer: As she begins the battle, the "mother" cell divides; one "daughter" takes up the fight while the other holds a memory of the invader for an even better response the next time.

The discovery earned a spot among Science magazine's top 10 breakthroughs of 2007. Even better, Reiner's peers on the Faculty of 1000 Biology Web site voted it No. 5 - of all time.

Other superlatives:

Children's Hospital of Philadelphia ranked No. 1 among pediatric hospitals twice - by U.S. News & World Report as well as the now-defunct Child Magazine. That comes as no surprise to Nadia Kadi, the little girl whose liver transplant in April was chronicled by The Inquirer. (Her most recent blood enzyme tests came back normal.)

A record number of bald eagles soared past Hawk Mountain in Berks County this fall, continuing a comeback that began with the banning of DDT in 1972. Then, 18 eagles were counted during the typical migration. The count this fall: 230. - Don Sapatkin