The causes of Alzheimer's disease are not clear. Its best-known characteristic is the slimy amyloid that grows into plaques covering the brain of patients, yet people can harbor a lot of that substance before losing memories. Scientists are observing what is inside living brains to observe for the first time while a toxic pair of plaques and tangles act together to weaken Alzheimer's disease. Those tangles may foresee budding symptoms, a result with implications for much better treatments.