Mass and weight are important ideas to understand when we talk about force and motion.

• Mass is how much stuff is in an object. It doesn't change, no matter where the object is. For instance, a 1 kg rock on Earth has the same mass as a 1 kg rock on the Moon.

• Weight is different. It measures how strong gravity pulls on that mass. We can figure out weight using this simple formula:

$$\text{Weight} = \text{Mass} \times \text{Gravity}$$

On Earth, gravity pulls down with a strength of about 9.8 meters per second squared ($9.8 \, \text{m/s}^2$). This means that a 1 kg rock weighs around 9.8 newtons ($9.8 \, \text{N}$).

If you take that rock into space, where there's less gravity, its weight would go down. But its mass would stay the same.