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In the movie Red Dragon, Will Graham pays a visit to Hannibal and discusses his new discovery:

• Will thinks the Chesapeake Killer is eating his victims.

Then when Hannibal goes away for a while, Will browses Hannibal's office and discovers a dictionary (French?) that has sweetbread written in it by Hannibal.

Then Will realizes Hannibal is the killer he has been looking for.

Why? What is the logical connection between sweetbread, the French dictionary and murders by Hannibal?

Graham finds Larousse Gastronomique, a French encyclopedia of gastronomy, and finds its bound bookmark to be located on a page describing a recipe that uses sweetbreads, organ meat from the thymus gland and pancreas.

Note that the term sweetbread is often used in a broader sense, as per Wikipedia:

a culinary name for the thymus (also called throat, gullet, or neck sweetbread) or the pancreas (also called heart, stomach, or belly sweetbread), especially of calf (ris de veau) and lamb (ris d'agneau), and, less commonly, of beef and pork. The "heart" sweetbreads are more spherical in shape, while the "throat" sweetbreads are more cylindrical in shape.

Various other glands used as food may also sometimes be called "sweetbreads," including the parotid gland ("cheek" or "ear" sweetbread), the sublingual glands ("tongue" sweetbreads or "throat bread") as well as ovary and testicles (cf. Rocky Mountain oyster, prairie oyster, or lamb fries).

Graham has just had a conversation with Lecter where he has discussed a cannibal killer who cuts out exactly those parts from humans, and expressed surprise at Lecter failing to notice this when he had helped him compile a profile of the killer.

Undoubtedly, he now flashes back to the conversation he's just had. Lecter asking "Have you shared this with the bureau?" Himself saying that "he's starting to think like the killer" and feeling that he's close to grasping some truth which seems to elude him.

And then he realizes that Lecter is the killer and that he's been sabotaging their investigation.

Sweetbread is an edible part of an animal, specifically...

Sweetbread is a culinary name for the thymus (also called throat, gullet, or neck sweetbread) or the pancreas (also called heart, stomach, or belly sweetbread), especially of calf (ris de veau) and lamb (ris d'agneau), and, less commonly, of beef and pork.1 The "heart" sweetbreads are more spherical in shape, while the "throat" sweetbreads are more cylindrical in shape.

^Wikipedia

Will is already starting to think like the killer and has already deduced that the killer is eating human body parts...he says to Lecter

"Then suddenly I had a flash of the third victim...

Darcy Taylor.

She was missing flesh from her back. And then it hit me.

Liver, kidney, tongue, thymus.

Every single victim lost some body part used in cooking."

AND he finds it odd that Lecter hasn't come to the same conclusion...

"You're the best forensic psychiatrist I know... and somehow, in all our time together... this possibility never occurred to you."

Then on perusing Lecter's collection of rare books, he comes across a French cooking book in which, apparently, Lector has written the word "sweetbreads".

Basically, at that point, he puts it all together.