A force is the push and pull that causes objects to undergo change. Everything in existence is affected by forces all of the time - whether it's the pull of gravity, the drag of air resistance and friction, or the push of upthrust. It's amazing that we cannot see forces, despite them being everywhere. So how do we know they exist? Well, we can see the effects they have, like the Earth orbiting the Sun because of the force of gravity, or a needle in a compass moving due to the force of magnetism.