John Preskill, while talking about taking a quantum mechanics class with the late John Wheeler, mentions this story:

Later, at a student-faculty lunch, Wheeler seemed troubled. He had been asked to explain the essence of quantum mechanics in five words or less, and was stumped. Frank Calaprice, a nuclear physicist within earshot, interjected helpfully, “What we expect to measure?” I was silent.

The question was absurd. It was fascinating. I still think about it. I can’t answer it.

This idea–explain quantum mechanics in five words–has captured the imagination of people. In the the blog comments, a number of people chime in with their thoughts:

My attempt at non-technical answer is: Quantum mechanics is nature's casino.

Sean Carroll, via Michael Nielsen, picked up on the idea and posted this Twitter challenge:

Which has led to a flood of tweets trying to summarize quantum mechanics. Here are my favourites so far:

How would you explain quantum mechanics in five words or less?