If you haven't experienced it yet, give your child a fresh bar of soap that has been dipped in the water. It's pretty hilarious. Make sure you say "whoops" as it slips out of your hands into your child's. It seems to work better with a bigger bar of soap. I'm assuming that this is since there is more slippery surface and it's harder for small hands to grab in the first place.
Theory: Reduced Friction Surface
Friction is caused when items rub together. The closest thing we get to frictionless surfaces in the touchable world is rubbing ice-on-ice. Soap is a slippery surface with reduced friction.
What other items are slippery to hold?
**Sorry I don't have a video. I wasn't expecting the reaction I got when I first gave J the soap and I didn't feel comfortable leaving him in the tub while I grabbed the video camera. Trust me, it was definitely good. I hope you get a similar reaction.
***We were having bathing issues a few months ago and the novelty of trying to hold onto the soap helped us get J into the tub for a few weeks, until the novelty wore off.