It's acoustic vibrations in the air, little waves of energy in the air that tickle our eardrum.

An intriguing clue comes from research in neuroscience that explains why it is impossible to tickle yourself.

He'd you can tickle him. When I put my hand up, Ha, ha, ha, ha and that's how we sold him.

I think of Shakespeare's Shylock If you prick us, do we not bleed, and if you tickle us, do we not laugh?

For a few years we managed to circumvent that part of our brains that told us we couldn t tickle ourselves to greater wealth.

Even one like, say, Tickle Me Elmo which, despite its ingenuity, really only does one thing, whereas boxes offer an infinite number of choices.

Because the brain of a person trying to tickle herself anticipates the sensations that will be caused by her fingers a process performed within the cerebellum the sensation no longer tickles.

So, as you can see in this prototype, gigantic piano strings, each string is controlled with a little robotic element either little bows that stroke the strings, propellers that tickle the strings, acoustic signals that vibrate the strings. We also have an army of robots on stage.