Achilles' heel

Achilles' heel is a historical body part acquired by the Professor at an auction. It is a symbol for deadly weaknesses.

Trivia

 * The heel we saw in "The Duh-Vinci Code" could have been preserved like many other body parts, such as the head. If this is true, we can assume it was somehow removed from . It's also possible that the Professor was ripped off, as Achilles is a mythological character.
 * Achilles' heel could also refer to the in general, in which case it is meant to be Leonardo's Achilles heel. As Leonardo was still alive and intact at that point, this is still a rip off.