A physicist asked his 6 year old son to imagine standing on top of a big ball in outer space. Then, he told his son, he looks over the edge, and there is a person on the other side of the ball, upside down, with feet on the bottom of the ball. He asked his son: will that upside-down person fall off the ball? His son thought about it, and said "If the other person looks at me, he sees my feet, and I'm upside down to him. I'm not falling off, so he doesn't fall off".
Pretty intelligent answer. We would be tempted to answer that gravity holds us together, and we will easily miss this symmetry.
- via Quora