This thread on stirring natural peanut butter in the jar is an example of why I love Ask Metafilter, the detailed sincere answers to a seemingly trivial question. I myself dump the contents of a new jar of separated peanut butter into a large bowl and use a potato masher to mix everything back together, then I refrigerate the peanut butter to keep it from separating again. But, my method makes more dishes. People on this thread have a bunch of techniques: hit the open jar with a short burst in the microwave, pierce the solid bits and let the oil drip down, store the jar upside down for a few days before opening, and even pointed to this nifty (if uni-task-y) peanut butter mixing device. Somebody even reported back on trying out the upsidedown storage method. Awesome.