Silicon Valley Event On Machine Learning Tackles The Latest Riddles Vexing AI Self-Driving Cars

There's a child's riddle that asks you to indicate what can be held in your left hand and yet cannot be held in your right hand. Take a moment to ponder this riddle. Your first thought might be that anything that could be held in your left hand should also be able to be held in your right hand, assuming of course that there's no trickery involved. One trick might be that you could hold your right hand in your left hand, but that you cannot presumably "hold" your right hand in your right hand since your right hand is your right hand. Another trick might be that your right hand is perchance weaker than your left hand, thus if an object was heavy, potentially you could hold it in your left hand, but you could not do so with your less powerful right hand. If we eliminate all the trickery potential answers, what else remains?