Drop anything — a ball, a spoon, an apple — and it falls down. Every single time. The pull that does it is gravity: the Earth tugging on everything around it, all the time, always towards the ground.

Gravity is an invisible pull, a bit like a magnet that works on everything at once. We even have a name for how hard gravity pulls on an object — its weight. Press play and watch the apple fall.

The more stuff (mass) something has, the harder gravity pulls it — so its weight arrow is longer. Drag the mass up and watch the downward weight arrow grow.