As great as Galileo was, Isaac Newton was the one who really hit it out of the park with this idea: he said that gravity is not just about the Earth pulling things, but rather that everything that has mass creates a gravitational force.
Consider an apple and the planet Earth.
Newton said that the apple is pulling on the Earth in exactly the same way as the Earth is pulling on the apple.
Then, you might ask, “Why doesn’t the Earth fall up to the apple?” Well, the answer to that is...
Just not very much. Actually, it’s an immeasurably small distance.
So, how can the apple and the Earth react to equal forces so differently?