Blood Transfusion

Tom's girlfriend Caroline was about to die.  She needed a blood transfusion in order to save her life.  It just so happened that she and Tom had the same blood type, so Tom gallantly offered her some of his blood.  She lived.  

A few months later, Tom and Caroline broke up.  Tom was pissed, and felt like his blood tranfusion (cruelly) had been a waste.  So before he walked away from her he said, "Hey, I want my blood back."

"Okay" she said, "hold on one second."  Then she went into the bathroom.  

When she came back out again she was carrying a tampon.  "Here," she said, "you want your blood back?  I'll pay monthly."