Cameron Moran of Queenstown asks :-

What causes gravity?

Lara Wilcocks and Kirsten Franklin, physicists at Otago University, responded.

If you throw a ball up in the air, it slows down and then falls back towards the ground speeding up. If we do the same thing on the moon, the ball takes longer to slow down and longer to speed up. The ball behaves differently because the Moon is smaller than the Earth – the Earth has more mass. Saying how much mass something has is like saying how much stuff is in it. A scientist called Sir Isaac Newton figured out that everything that has mass is pulled towards every other thing that has mass, just because of their masses. We call this attraction "gravity" – so the mass of things is what causes the force of gravity between them.

Your weight is just the mass of the Earth pulling on your mass. It is just not really noticeable for things are small and have small masses. You won't see your pen fly towards your head, because the pen and your head are really small (compared to the Earth!) so that the pull of gravity between them is really weak. But when you have something really, really big like the planet Earth, the attraction between all of the Earth's mass and you becomes very, very strong, and so you are pulled towards the middle of the Earth.

The mass of the Earth is about six trillion trillion kilograms. If you try to leave the Earth's surface – by jumping up into the air for example, you get pulled back down again to the Earth by the Earth's mass.