I was looking out of the window on a flight out of Madrid, just daydreaming, when I came up with the idea of the Earth sandwich. I was on my way back to New York, which felt like the other side of the world compared to the Spanish landscape I could see below. The stewardesses hadn't started serving food but I just thought, what if somebody created the biggest sandwich in the world - with Earth as the filling in the middle?

By the time I landed in America, the Earth sandwich was already morphing into a challenge in my mind. At the time, I was running a daily video blog and I used to ask people to do crazy things - and this was certainly a challenge. I knew people were always looking for wacky ideas but I had no idea the Earth sandwich would take off like it did.

The antipodes of any spot on the planet is the place diametrically opposite it, on the other side of the world. For people to achieve an Earth sandwich, they would have to find somebody to place a piece of bread at precisely the right location, on the opposite side of the Earth, at exactly the same time as they did.