Simple: claim amnesia and act pleasent.
Besides, sanity is a subjective term anyway. I usually define being insane as being able to think rationally, but this does not hold up entirely well for how we treat people nowadays. For example, we generally consider psychopaths to be insane (do note that I wrote psychopathic, not psychotic; people too often confuse the two), however they are more often than not able to think perfectly rationally; they just lack certain emotions that normal people have, namely empathy. This, to me at least, is very worrisome. If we are able to lock up people and label them insane simply due to what we don't consider to be "healthy" emotions, what's to stop each and every one of us from getting put away? It's not about being "sane," it's about trying to stay on the good side of people with authority. [/rant]