When Mr. Burns needs a blood transfusion, Homer is pleased to learn that Bart has the same rare blood type, Bart donates his blood and saves Mr. Burns, but Homer is upset to find that Mr. Burns sends him nothing more than a thank you note. Homer writes an angry letter in responce, but then desides not to send it, but then he discovers that Bart had already mailed it. Homer tries his best to recover the letter, but fails, and Mr. Burns reads the letter and vows revenge. Smithers finally convinces Mr. Burns that the Simpsons have indeed done him a favor and, in gratitude, Mr. Burns buys them a rare Olmec Indian Head. I thought this episode was pretty funny. Also the Olmec Indian Head was in one of the "Simpsons Comics"