As Harry felt the Sorcerer's stone fall into his pocket and saw himself wink in the mirror of Erised he couldn't help but think that Quirrel could easily find it. In a blind panic he pushed his hand against the stone and tried to hide it against his hip when he felt it sink into him. He looked down and saw his crotch glow red, then pulse for a second. He heard Voldemort howl "He has absorbed the stone!" When he awoke in the hospital wing he found himself with a permenant slightly glowing erection. That gave eternal life.