for inflammation. Piece 1 is the immediate cause of Alzheimer's disease: the appearance of insoluble "plaques" made of a small protein called amyloid beta (A-beta for short) inside brain cells. These plaques block the traffic of molecules in the cells. Eventually another small protein, called tau, also starts to crystallize in this way to form "tangles." Both symptoms are diagnostic of Alzheimer's, and similar ones characterize other neurological syndromes such as Parkinson's and Creutzfeldt-Jakob's. Puzzle piece 2 is the APOE gene on chromosome 19, long known as a powerful influence on whether you will get Alzheimer's disease. Having two copies of the 4 version of the gene makes you 20 times more likely than average to get the symptoms before the age of 75. (Having at least one copy of the 2 version makes you less likely than average to get the symptoms.) One of APOE's jobs is to break down plaques, and the 4 version is inefficient at this task. My latest Mind and Matter column in the Wall Street Journal is about trying to evolve, rather than ordain, solution to obesity Sometimes we find it easy to identify a problem and impossible to think of a solution. Obesity is a good example. Almost everybody agrees that it is a growing burden on health systems and that it requires urgent attention from policy makers. But almost everybody also agrees that no policy for reducing obesity is working. But for some time I have been noticing that the correlations between