Pick up a bottle. Within a nanosecond you'll instinctively apply the exact pressure needed to keep it in your hand without breaking it. You'll also figure out without checking the contents if it's full, partially full, or empty. This sense is called proprioception, and we're only aware of it because of Dr. Oliver Sacks' research into patients who were lacking this very sense.
It's no wonder that it took us this long to help our future mechanical masters this extrasensory awareness. We have budding roboticist Jivko Sinapov. His summer internship at research lab Willow Garage involved giving their resident robot, the PR2, the ability to detect the contents of a bottle without visual input.
First, Sinapov used a proprioceptive subroutine which allowed the robot to detect how much effort it was putting in to lifting an object. He then let the PR2 learn the difference between empty and full by having it lift bottles with varying amounts of contents. Once the robot amassed enough data, it was able to determine, based on previous experience, whether a particular bottle was empty or full.
Finally, Sinapov built a sorting routine which tells PR2 to place empty bottles in a trash bin, while arranging full ones on the other side of a table. The result? A 100% success rate. Future improvements to PR2 involve developing his proprioception to determine if boxes are empty with a slight nudge. Looks like PR2's ready to take over your mail-sorting job at the post office, if not the world.